Add shell of rvm instructions for mac

Prior to this commit, we didn't even have a list of the steps that would
be required for being able to contribute to the website. This commit
adds the shell of the steps required for contributing on a modern-ish
version of Mac OS X.
Add Xcode details

Add Xcode and Command Line Tools details and screenshot.
Document postgresql requirement for contributing
Flesh out most of the installation instructions
Add in-page links to the overview
Merge pull request #1 from AndrewLorente/add_contribution_requirements_instructions

Document postgresql requirement for contributing
Merge pull request #1 from jeffweiss/add_contribution_requirements_instructions

Add setup instructions for OSX