Update index.rst with link to source of stable release

The current live version of these docs on http://flask.pocoo.org/ refers to the master branch, which is actively being used for development of both flask and flaskr, and so the latest stable release (0.10.1) doesn't work with the master branch of flaskr, even though that's what's linked here. So it's probably a good idea to include this alternative link for the majority of users.