docs(README): add downloads badge

Author: whtsky

README.rst

@@ -18,6 +18,9 @@ A fast bencode implementation in Cython supports both Python2 & Python3 .
 .. image:: https://codecov.io/gh/whtsky/bencoder.pyx/branch/master/graph/badge.svg
     :alt: Codecov Coverage
     :target: https://codecov.io/gh/whtsky/bencoder.pyx
+.. image:: https://pypistats.com/badge/bencoder-pyx.png
+    :alt: bencoder-pyx PyPI Downloads
+    :target: https://pypistats.com/package/bencoder-pyx
 
 Install
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