Mention system requirements in README files

[JedGrabman]

I ran into issues while attempting to run through the setup instructions in the validator/README.md on a Windows machine while using Git Bash:

1. I needed to use env/scripts in place of env/bin throughout
2. pip install ../validator errored out:

   Collecting fiona
   Using cached Fiona-1.8.17.tar.gz (1.3 MB)
   ERROR: Command errored out with exit status 1:
    command: 'c:\users\jed\documents\github\covidcast-indicators\usafacts\env\scripts\python.exe' -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'C:\\Users\\Jed\\AppData\\Local\\Temp\\pip-install-e04foeth\\fiona\\setup.py'"'"'; __file__='"'"'C:\\Users\\Jed\\AppData\\Local\\Temp\\pip-install-e04foeth\\fiona\\setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base 'C:\Users\Jed\AppData\Local\Temp\pip-pip-egg-info-tgmo6t71'
        cwd: C:\Users\Jed\AppData\Local\Temp\pip-install-e04foeth\fiona\
   Complete output (1 lines):
   A GDAL API version must be specified. Provide a path to gdal-config using a GDAL_CONFIG environment variable or use a GDAL_VERSION environment variable.
   ----------------------------------------
   ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

I was able to solve the issue by following this StackOverflow answer and running Git Bash as an administrator

We should make a note of the issues in the README and ideally make a change to the package so it won't error out when installing.

[krivard]

All packages in covidcast-indicators assume a linux environment. There are no plans to add or maintain cross-platform support, but we should probably mention system requirements in the README files.

[chinandrew]

Why does validator even need Fiona/geopandas?

[JedGrabman]

@chinandrew I think it's just because it imports covidcast, which pulls it in as a dependency for creating graphs.