Added RELEASE_NOTES.txt.

Reviewed by: kiyan
CHOP-59. Adds SongRecommender and test suite.

This class provids and example of joining data from two tables in
a scalding flow.

Reviewed By: Kiyan Ahmadizadeh
Fixes comment on recommendedSong pipe.

Reviewed By: Kiyan Ahmadizadeh
Fixed SongRecommender.express to not require cli args.

Reviewed by: kiyan