Link to stable version of documentation from README

Author: ddemidov

README.rst

@@ -4,7 +4,7 @@ Python language bindings for ev3dev
 .. image:: https://travis-ci.org/rhempel/ev3dev-lang-python.svg?branch=master
     :target: https://travis-ci.org/rhempel/ev3dev-lang-python
 .. image:: https://readthedocs.org/projects/python-ev3dev/badge/?version=stable
-    :target: http://python-ev3dev.readthedocs.org/en/latest/?badge=stable
+    :target: http://python-ev3dev.readthedocs.org/en/stable/?badge=stable
     :alt: Documentation Status
 
 A Python3 library implementing an interface for ev3dev_ devices,