improve Windows instructions

[nickolay]

As someone who didn't develop on Windows for a long time, I had a few problems building this:

• First I managed to miss the installation instructions: since the README appeared to give some advice on building but did not mention Windows (except in the first sentence) I assumed I had to figure it out by myself.
• Next I found out I had to install cmake (which didn't add itself to PATH)

• Finally (I think because I built FreeTDS with the default settings, resulting in x86_64 build, and the attempted to pip install ctds in a 32-bit python venv) I had some fun trying to figure out the LNK2001: unresolved external symbol errors:

  C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\BIN\link.exe /nologo /INCREMENTAL:NO /LTCG /DLL /MANIFEST:EMBED,ID=2 /MANIFESTUAC:NO /LIBPATH:C:\Users\Nickolay\ctds/build/lib /LIBPATH:c:\users\nickolay\ctds\pyenv\libs /LIBPATH:C:\Users\Nickolay\AppData\Local\Programs\Python\Python36-32\libs /LIBPATH:C:\Users\Nickolay\AppData\Local\Programs\Python\Python36-32 /LIBPATH:c:\users\nickolay\ctds\pyenv\PCbuild\win32 "/LIBPATH:C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\LIB" "/LIBPATH:C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\ATLMFC\LIB" "/LIBPATH:C:\Program Files (x86)\Windows Kits\10\lib\10.0.17134.0\ucrt\x86" "/LIBPATH:C:\Program Files (x86)\Windows Kits\NETFXSDK\4.6.1\lib\um\x86" "/LIBPATH:C:\Program Files (x86)\Windows Kits\10\lib\10.0.17134.0\um\x86" sybdb.lib ct.lib shell32.lib ws2_32.lib /EXPORT:PyInit__tds build\temp.win32-3.6\Release\src\ctds\connection.obj build\temp.win32-3.6\Release\src\ctds\cursor.obj build\temp.win32-3.6\Release\src\ctds\parameter.obj build\temp.win32-3.6\Release\src\ctds\pyutils.obj build\temp.win32-3.6\Release\src\ctds\tds.obj build\temp.win32-3.6\Release\src\ctds\type.obj /OUT:build\lib.win32-3.6\_tds.cp36-win32.pyd /IMPLIB:build\temp.win32-3.6\Release\src\ctds\_tds.cp36-win32.lib
  Creating library build\temp.win32-3.6\Release\src\ctds\_tds.cp36-win32.lib and object build\temp.win32-3.6\Release\src\ctds\_tds.cp36-win32.exp
  connection.obj : error LNK2001: unresolved external symbol _bcp_batch
  connection.obj : error LNK2001: unresolved external symbol _dbclose
  connection.obj : error LNK2001: unresolved external symbol _bcp_getl
  connection.obj : error LNK2001: unresolved external symbol _dbsetuserdata
  connection.obj : error LNK2001: unresolved external symbol _dbcancel
  connection.obj : error LNK2001: unresolved external symbol _bcp_options
  connection.obj : error LNK2001: unresolved external symbol _dbspid
  connection.obj : error LNK2001: unresolved external symbol _dbsetlbool
  connection.obj : error LNK2001: unresolved external symbol _dbtds
  connection.obj : error LNK2001: unresolved external symbol _dbsetlogintime
  connection.obj : error LNK2001: unresolved external symbol _dbsetlname
  connection.obj : error LNK2001: unresolved external symbol _bcp_init
  connection.obj : error LNK2001: unresolved external symbol _dbsqlexec
  connection.obj : error LNK2001: unresolved external symbol _dbsetopt
  connection.obj : error LNK2001: unresolved external symbol _dbdead
  connection.obj : error LNK2001: unresolved external symbol _dbcmd
  connection.obj : error LNK2001: unresolved external symbol _dbname
  connection.obj : error LNK2001: unresolved external symbol _bcp_done
  connection.obj : error LNK2001: unresolved external symbol _dbuse
  connection.obj : error LNK2001: unresolved external symbol _dbresults
  connection.obj : error LNK2001: unresolved external symbol _dbsettime
  connection.obj : error LNK2001: unresolved external symbol _dbloginfree
  connection.obj : error LNK2001: unresolved external symbol _dbsetlversion
  connection.obj : error LNK2001: unresolved external symbol _bcp_sendrow
  connection.obj : error LNK2001: unresolved external symbol _tdsdbopen
  connection.obj : error LNK2001: unresolved external symbol _dbnextrow
  connection.obj : error LNK2001: unresolved external symbol _dbgetuserdata
  connection.obj : error LNK2001: unresolved external symbol _dblogin
  cursor.obj : error LNK2001: unresolved external symbol _dbretlen
  cursor.obj : error LNK2001: unresolved external symbol _dbdatlen
  cursor.obj : error LNK2001: unresolved external symbol _dbcount
  cursor.obj : error LNK2001: unresolved external symbol _dbadlen
  cursor.obj : error LNK2001: unresolved external symbol _dbcolinfo
  cursor.obj : error LNK2001: unresolved external symbol _dbnumrets
  cursor.obj : error LNK2001: unresolved external symbol _dbrettype
  cursor.obj : error LNK2001: unresolved external symbol _dbalttype
  cursor.obj : error LNK2001: unresolved external symbol _dbretstatus
  cursor.obj : error LNK2001: unresolved external symbol _dbrpcinit
  cursor.obj : error LNK2001: unresolved external symbol _dbsqlok
  cursor.obj : error LNK2001: unresolved external symbol _dbretdata
  cursor.obj : error LNK2001: unresolved external symbol _dbsqlsend
  cursor.obj : error LNK2001: unresolved external symbol _dbrpcsend
  cursor.obj : error LNK2001: unresolved external symbol _dbdata
  cursor.obj : error LNK2001: unresolved external symbol _dbretname
  cursor.obj : error LNK2001: unresolved external symbol _dbadata
  cursor.obj : error LNK2001: unresolved external symbol _dbnumcols
  cursor.obj : error LNK2001: unresolved external symbol _dbfreebuf
  parameter.obj : error LNK2001: unresolved external symbol _bcp_bind
  parameter.obj : error LNK2001: unresolved external symbol _dbconvert_ps
  parameter.obj : error LNK2001: unresolved external symbol _dbrpcparam
  tds.obj : error LNK2001: unresolved external symbol _dberrhandle
  tds.obj : error LNK2001: unresolved external symbol _ct_config
  tds.obj : error LNK2001: unresolved external symbol _dbinit
  tds.obj : error LNK2001: unresolved external symbol _dbexit
  tds.obj : error LNK2001: unresolved external symbol _dbmsghandle
  type.obj : error LNK2001: unresolved external symbol _dbconvert
  type.obj : error LNK2001: unresolved external symbol _dbdatecrack
  build\lib.win32-3.6\_tds.cp36-win32.pyd : fatal error LNK1120: 57 unresolved externals
  error: command 'C:\\Program Files (x86)\\Microsoft Visual Studio 14.0\\VC\\BIN\\link.exe' failed with exit status 1120

Once I got through building it, actually using the library was a breeze, so I thought I would document my mistakes to help others enjoy it.

Thanks for making this excellent library!

[codecov-io]

Codecov Report

Merging #30 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #30   +/-   ##
=======================================
  Coverage   93.71%   93.71%           
=======================================
  Files           8        8           
  Lines        2593     2593           
  Branches        8        8           
=======================================
  Hits         2430     2430           
  Misses        163      163

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