Merge update-readme-6953-2: Update the README for new docs

Author: hawkowl
Reviewers: exarkun, adiroiban, glyph, khorn
Fixes: #6953

Update the README to point to web-hosted docs, rather than local ones, and add a reference to #twisted.web.

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@43084 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb