Add contribution documentation

Hello @don-neufeld, 

Please review the following commits I made in branch 'es-contribution-docs'.

In particular, are we cool with moving the OCLA info from the readme to contributing.md? I think it fits better here.

94efef71905b701476e35a7bb957995e332d0d34 (2013-09-06 14:44:15 -0700)
Add CONTRIBUTING.md
Displayed on the GitHub PR screen: https://github.com/blog/1184-contributing-guidelines

Reference contributing page from the readme.


R=@don-neufeld