[SPARK-6806] [SPARKR] [DOCS] Add a new SparkR programming guide

This PR adds a new SparkR programming guide at the top-level. This will be useful for R users as our APIs don't directly match the Scala/Python APIs and as we need to explain SparkR without using RDDs as examples etc.

cc rxin davies pwendell

cc cafreeman -- Would be great if you could also take a look at this !

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #6490 from shivaram/sparkr-guide and squashes the following commits:

d5ff360 [Shivaram Venkataraman] Add a section on HiveContext, HQL queries
408dce5 [Shivaram Venkataraman] Fix link
dbb86e3 [Shivaram Venkataraman] Fix minor typo
9aff5e0 [Shivaram Venkataraman] Address comments, use dplyr-like syntax in example
d09703c [Shivaram Venkataraman] Fix default argument in read.df
ea816a1 [Shivaram Venkataraman] Add a new SparkR programming guide Also update write.df, read.df to handle defaults better