Document __main__.__requires__

Also modernises the initial overview for pkg_resources, and helps
make it clear that it covers anything with an egg-info directory,
not just egg files.

--HG--
branch : docs_update_for_requires
Add back a more descriptive intro

--HG--
branch : docs_update_for_requires
Typo fix

--HG--
branch : docs_update_for_requires
Merged in ncoghlan/setuptools/docs_update_for_requires (pull request #14)

Document __main__.__requires__