Add a Community Projects page

This adds a new page to the docs listing community projects -- those created outside of Apache Spark that are of interest to the community of Spark users.   Anybody can add to it just by submitting a PR.

There was a discussion thread about alternatives:
* Creating a Github organization for Spark projects -  we could not find any sponsors for this, and it would be difficult to organize since many folks just create repos in their company organization or personal accounts
* Apache has some place for storing community projects, but it was deemed difficult to work with, and again would be some permissions issues -- not everyone could update it.

Author: Evan Chan <velvia@gmail.com>

Closes #2219 from velvia/community-projects-page and squashes the following commits:

7316822 [Evan Chan] Point to Spark wiki: supplemental projects page
613b021 [Evan Chan] Add a few more projects
a85eaaf [Evan Chan] Add a Community Projects page