Merge pull request #950 from mnaberez/readme-link

Fix link to README broken in #937

Author: mark-edgeworth

README.rst

@@ -12,7 +12,7 @@ Mbed CLI supports both Git and Mercurial repositories, so you'll also need to in
 
 Get Started
 ===========
-The best way to get started is to `read the documentation on GitHub <https://github.com/ARMmbed/mbed-cli/blob/master/README.md>`_.
+The best way to get started is to `read the documentation on GitHub <https://github.com/ARMmbed/mbed-cli/blob/master/README.rst>`_.
 
 License
 =======