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

README.md

@@ -13,7 +13,8 @@ and Spark Streaming for stream processing.
 ## Online Documentation
 
 You can find the latest Spark documentation, including a programming
-guide, on the [project web page](http://spark.apache.org/documentation.html).
+guide, on the [project web page](http://spark.apache.org/documentation.html)
+and [project wiki](https://cwiki.apache.org/confluence/display/SPARK).
 This README file only contains basic setup instructions.
 
 ## Building Spark

docs/index.md

@@ -112,6 +112,7 @@ options for deployment:
 **External Resources:**
 
 * [Spark Homepage](http://spark.apache.org)
+* [Spark Wiki](https://cwiki.apache.org/confluence/display/SPARK)
 * [Mailing Lists](http://spark.apache.org/mailing-lists.html): ask questions about Spark here
 * [AMP Camps](http://ampcamp.berkeley.edu/): a series of training camps at UC Berkeley that featured talks and
   exercises about Spark, Spark Streaming, Mesos, and more. [Videos](http://ampcamp.berkeley.edu/3/),