[SPARK-2843][MLLIB] add a section about regularization parameter in ALS

@atalwalkar @srowen
This technique was used by many groups in the Netflix Prize (see section 5 of this paper for some more references: http://www.cs.toronto.edu/~rsalakhu/papers/weighted_tc.pdf).  Perhaps this sentence should be changed to "This approach is discussed in the following paper..."