Added description of ChiSqSelector and few words about feature selection in general. I could add a code example, however it would not look reasonable in the absence of feature discretizer or a dataset in the `data` folder that has redundant features.

Author: Alexander Ulanov <nashb@yandex.ru>

Closes #4709 from avulanov/SPARK-5912 and squashes the following commits:

19a8a4e [Alexander Ulanov] Addressing reviewers comments @jkbradley
58d9e4d [Alexander Ulanov] Addressing reviewers comments @jkbradley
eb6b9fe [Alexander Ulanov] Typo
2921a1d [Alexander Ulanov] ChiSqSelector example of use
c845350 [Alexander Ulanov] ChiSqSelector docs