Addition of system dependant .egg files - Documentation & change request

[orsenthil]

Recently I desired to include library 'urwid' into pants.pex. I ran into multiple troubles while including it and finally came to know that since urwid builds into system depended .egg files, I will have build them and them check them into pants/third_party/python/dist/ in the binaries branch and rebuild the index.

It will be a good idea to document this for future reference.

And better will be:

Pants utilizes the python-standard way where in distribute library can just build those eggs during pants build time by downloading from pypi.python.org

[jsirois]

Agreed on both points.