[SPARK-3227] [mllib] Added migration guide for v1.0 to v1.1

The only updates are in DecisionTree.

CC: mengxr

Author: Joseph K. Bradley <joseph.kurata.bradley@gmail.com>

Closes #2146 from jkbradley/mllib-migration and squashes the following commits:

5a1f487 [Joseph K. Bradley] small edit to doc
411d6d9 [Joseph K. Bradley] Added migration guide for v1.0 to v1.1.  The only updates are in DecisionTree.