Add coverage badge in README.

[ci skip]

Author: whtsky

README.rst

@@ -9,6 +9,8 @@ A fast bencode implementation in Cython supports both Python2 & Python3 .
     :target: https://ci.appveyor.com/project/whtsky/bencoder.pyx
 .. image:: https://semaphoreci.com/api/v1/whtsky/bencoder-pyx/branches/master/shields_badge.svg
     :target: https://semaphoreci.com/whtsky/bencoder-pyx
+.. image:: https://codecov.io/gh/whtsky/bencoder.pyx/branch/master/graph/badge.svg
+    :target: https://codecov.io/gh/whtsky/bencoder.pyx
 
 Install
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