Clarified Python version suitable for development. Removed reference to Lion to be more generic OS X.
Created top level how to contribute doc as suggested in Issue: 263
Merge pull request #389 from willingc/doc-263

Created top level how to contribute doc as suggested in Issue: 263
Add two free beginner's books to the list
Merge pull request #388 from willingc/doc-359

Issue 359: Proposed clarification for OS X Python Installation
Merge pull request #392 from asweigart/patch-2

Add two free beginner's books to the list