https://issues.apache.org/jira/browse/SPARK-7272

Author: Vincenzo Selvaggio <vselvaggio@hotmail.it>

Closes #6219 from selvinsource/mllib_pmml_model_export_SPARK-7272 and squashes the following commits:

c866fb8 [Vincenzo Selvaggio] Update mllib-pmml-model-export.md
1beda98 [Vincenzo Selvaggio] [SPARK-7272] Initial user guide for pmml export
d670662 [Vincenzo Selvaggio] Update mllib-pmml-model-export.md
2731375 [Vincenzo Selvaggio] Update mllib-pmml-model-export.md
680dc33 [Vincenzo Selvaggio] Update mllib-pmml-model-export.md
2e298b5 [Vincenzo Selvaggio] Update mllib-pmml-model-export.md
a932f51 [Vincenzo Selvaggio] Create mllib-pmml-model-export.md