[SPARK-6889] [DOCS] CONTRIBUTING.md updates to accompany contribution doc updates

Part of the SPARK-6889 doc updates, to accompany wiki updates at https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark

See draft text at https://docs.google.com/document/d/1tB9-f9lmxhC32QlOo4E8Z7eGDwHx1_Q3O8uCmRXQTo8/edit#

Author: Sean Owen <sowen@cloudera.com>

Closes #5623 from srowen/SPARK-6889 and squashes the following commits:

03773b1 [Sean Owen] Part of the SPARK-6889 doc updates, to accompany wiki updates at https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark

CONTRIBUTING.md

 ## Contributing to Spark
 
-Contributions via GitHub pull requests are gladly accepted from their original
-author. Along with any pull requests, please state that the contribution is
-your original work and that you license the work to the project under the
-project's open source license. Whether or not you state this explicitly, by
-submitting any copyrighted material via pull request, email, or other means
-you agree to license the material under the project's open source license and
-warrant that you have the legal authority to do so.
+*Before opening a pull request*, review the 
+[Contributing to Spark wiki](https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark). 
+It lists steps that are required before creating a PR. In particular, consider:
+
+- Is the change important and ready enough to ask the community to spend time reviewing?
+- Have you searched for existing, related JIRAs and pull requests?
+- Is this a new feature that can stand alone as a package on http://spark-packages.org ?
+- Is the change being proposed clearly explained and motivated?
 
-Please see the [Contributing to Spark wiki page](https://cwiki.apache.org/SPARK/Contributing+to+Spark)
-for more information.
+When you contribute code, you affirm that the contribution is your original work and that you 
+license the work to the project under the project's open source license. Whether or not you 
+state this explicitly, by submitting any copyrighted material via pull request, email, or 
+other means you agree to license the material under the project's open source license and 
+warrant that you have the legal authority to do so.