Complete description for index.rst in doc

[bobleesj]

It appears that the documentation is incomplete:

Welcome to SNMF's Documentation!
====================================

``SNMF``: This library implements the stretched non negative matrix factorization (sNMF) and sparse stretched NMF
(ssNMF) algorithms described in ...

[sbillinge]

yes, this package itself is incomplete. I am not sure it is even working. under development.

Finishing it is not part of the summer project, so please just make an issue when you find something that needs fixing but move on. This is an important package so we will have another project ahead to finish it.

[john-halloran]

Closed by #187.