[SPARK-5539][MLLIB] LDA guide

This is the LDA user guide from jkbradley with Java and Scala code example.

Author: Xiangrui Meng <meng@databricks.com>
Author: Joseph K. Bradley <joseph@databricks.com>

Closes #4465 from mengxr/lda-guide and squashes the following commits:

6dcb7d1 [Xiangrui Meng] update java example in the user guide
76169ff [Xiangrui Meng] update java example
36c3ae2 [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into lda-guide
c2a1efe [Joseph K. Bradley] Added LDA programming guide, plus Java example (which is in the guide and probably should be removed).