README improvements

[steven-cutting]

• README fixes
  ** Include reference to contribute guide in READEME. Some people have had a hard time finding their way from the repo to the contribution guide.
  ** In the "installation from source" section, refer to the actual section in the docs, instead of to the general website
  ** Update "Discussion and Development" section: remove reference to scipy mailing list, but add one to pandas-dev mailing list

[steven-cutting]

I'm fixing this now.