message: add link to readthedocs
diff: # JavaScript XMPP Client 4.0
  [![Build Status](https://travis-ci.org/jsxc/jsxc.svg?branch=master)](https://travis-ci.org/jsxc/jsxc)
  -[![Dependency Status](https://dependencyci.com/github/jsxc/jsxc/badge)](https://dependencyci.com/github/jsxc/jsxc)
  +[![Documentation Status](https://readthedocs.org/projects/jsxc/badge/?version=latest)](https://jsxc.readthedocs.io/en/latest/?badge=latest)
  +
  Real-time chat app. This app requires an external XMPP server (openfire, ejabberd etc.).
  