Add Codecov badge to README

Author: thomasrockhu

README.rst

@@ -1,7 +1,7 @@
 Python Prompt Toolkit
 =====================
 
-|Build Status| |AppVeyor| |PyPI| |RTD| |License|
+|Build Status| |AppVeyor| |PyPI| |RTD| |License| |Codecov|
 
 .. image :: https://github.com/prompt-toolkit/python-prompt-toolkit/raw/master/docs/images/logo_400px.png
 
@@ -210,6 +210,10 @@ Special thanks to
 .. |License| image:: https://img.shields.io/github/license/prompt-toolkit/python-prompt-toolkit.svg
  :target: https://github.com/prompt-toolkit/python-prompt-toolkit/blob/master/LICENSE
 
+.. |Codecov| image:: https://codecov.io/gh/prompt-toolkit/python-prompt-toolkit/branch/master/graphs/badge.svg?style=flat
+ :target: https://codecov.io/gh/prompt-toolkit/python-prompt-toolkit/
+
+
 Other libraries and implementations in other languages
 ******************************************************