doc: Doc for new settings TAGS_FOR_FEED

New setting "TAGS_FOR_FEED" is introduced, default is []
If TAGS_FOR_FEED is set then it will generate feed only for
those tags.

Example usage
TAGS_FOR_FEED = ["thats", "awesome"]

Added documentation for my previous patch.
(Ref: https://github.com/getpelican/pelican/pull/1455)

Signed-off-by: Aravinda VK <mail@aravindavk.in>