doc: adding link to the future readthedocs documentation

README.md

@@ -5,7 +5,7 @@
 
 [![Build status](https://gitlab.mpcdf.mpg.de/sluka/BioEM/badges/master/build.svg)](https://gitlab.mpcdf.mpg.de/sluka/BioEM/commits/master)
 [![Code coverage](https://gitlab.mpcdf.mpg.de/sluka/BioEM/badges/master/coverage.svg?job=total_coverage)](http://sluka.pages.mpcdf.de/BioEM/)
-[![Doc]()]()
+[![Doc](https://readthedocs.org/projects/pip/badge/?version=stable)](http://bioem.readthedocs.io)
 [![License: GPL v3][license-badge]](License.txt)