[SPARK-17840][DOCS] Add some pointers for wiki/CONTRIBUTING.md in README.md...

[SPARK-17840][DOCS] Add some pointers for wiki/CONTRIBUTING.md in README.md and some warnings in PULL_REQUEST_TEMPLATE

## What changes were proposed in this pull request?

Link to contributing wiki in PR template, README.md

## How was this patch tested?

Doc-only change, tested by Jekyll

Author: Sean Owen <sowen@cloudera.com>

Closes #15429 from srowen/SPARK-17840.

.github/PULL_REQUEST_TEMPLATE

@@ -2,11 +2,9 @@
 
 (Please fill in changes proposed in this fix)
 
-
 ## How was this patch tested?
 
 (Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
-
-
 (If this patch involves UI changes, please attach a screenshot; otherwise, remove this)
 
+Please review https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark before opening a pull request.

README.md

@@ -97,3 +97,8 @@ building for particular Hive and Hive Thriftserver distributions.
 
 Please refer to the [Configuration Guide](http://spark.apache.org/docs/latest/configuration.html)
 in the online documentation for an overview on how to configure Spark.
+
+## Contributing
+
+Please review the [Contribution to Spark](https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark)
+wiki for information on how to get started contributing to the project.