Improve the file README

[WagnerGMD]

The principal purpose is to provide the instruction to build LIbTorrent because they are difficult to find for a lot of people !

From my point of view, you can close (or merge ?) the 230 PR because it doesn't provide :

• the complete instructions to install LIbTorrent (it's require the line autoreconf -fi but one full example, it's always better).
• neither the both example because the master and the latest release are different !
• usually the documentation are also include inside this file (specially true when we can't find it (an example : the wiki doesn't exist)).
• and for the record, the file autogen.sh didn't exist anymore (vanished a few years ago).