SPARK-971 [DOCS] Link to Confluence wiki from project website / documentation

This is a trivial change to add links to the wiki from `README.md` and the main docs page. It is already linked to from spark.apache.org.

Author: Sean Owen <sowen@cloudera.com>

Closes #3169 from srowen/SPARK-971 and squashes the following commits:

dcb84d0 [Sean Owen] Add link to wiki from README, docs home page