Update migration guide with breaking changes

Author: Nick Pentreath

docs/ml-guide.md

@@ -108,7 +108,12 @@ and the migration guide below will explain all changes between releases.
 
 ### Breaking changes
 
-There are no breaking changes.
+* The class and trait hierarchy for logistic regression model summaries was changed to be cleaner
+and better accommodate the addition of the multi-class summary. This is a breaking change for user
+code that casts a `LogisticRegressionTrainingSummary` to a
+` BinaryLogisticRegressionTrainingSummary`. Users should instead use the `model.binarySummary`
+method. See [SPARK-17139](https://issues.apache.org/jira/browse/SPARK-17139) for more detail 
+(_note_ this is an `Experimental` API).
 
 ### Deprecations and changes of behavior