New Article on Number of Partitions

@vidaha @mateiz @pwendell - please fact check for me! This is based on a support case I'm working on.

P.S. It seems easiest to review this straight from my master branch on github: https://github.com/patmcdonough/spark-knowledgebase/blob/master/troubleshooting/how_many_partitions_does_an_rdd_have.md
@mateiz @pwendell  - found it!

Also created SPARK-3580 (probably a good beginner task BTW).