Update SDK requirements. See 1c0c12dd.
Move installation instructions to separate file.

While I am aware that this basically reverts 3a968fcc, I now think that
installation instructions are important enough that they should have
their own section.

I am large, I contain multitudes.
Change headings to conform with RST specification.

Also gets rid of weird "Welcome to ... documentation!" text in heading.
Add docs on support and development. Fixes #109.
Fix syntax error in installation instructions.
Explicitly state package name in install docs.

This will hopefully decrease the number of people installing broken
packages ("facebook", "facebook-python-sdk", etc.) from PyPi.
Point CONTRIBUTING.rst at docs.

Duplication of information makes it more likely that documentation will
not be updated.
Update README.rst

It was unclear to me that the package was called 'facebook' at first glance.
Documentation updates.

This improves the existing documentation, but does not extend it
sufficiently. More improvement is still needed.