[summary] Add links to low-level descriptor documentation.

There's no place/documentation for any high-level descriptors just yet:
http://chatlogs.musicbrainz.org/musicbrainz-devel/2014/2014-11/2014-11-24.html#T16-11-06-969100
Merge pull request #43 from Freso/link-essentia-docs

Add links to low-level descriptor documentation to summary.html.