Rearrange into logical sections and add relevant links
This rearranges the Contributing page into more logical sections and provides some more useful links to wiki pages.  I think this should be the place where all relevant documentation on pandas development can be found.  For example, both this page and the pandas homepage mention `vbench` but the information on running tests was lacking and it turns out that the wiki has this information but it was difficult to find.  Hopefully this is a step in the right direction.

This looks like a nice improvement! Merging...
 @rockg Thanks a lot for this! Just a comment about the numpydoc reference, but maybe I missed it.
 It is at the top of the "Documentation / Commit Messages" section (I felt it was important).
 Ah, so I missed it! :-)
