Add Gitter badge (#45)

Author: gitter-badger

README.rst

@@ -20,3 +20,8 @@ and context managers.
 It is fully agnostic to ``async`` event loops and seamlessly works with
 ``asyncio``, third-party libraries such as ``trio``, as well as
 any custom ``async`` event loop.
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+.. image:: https://badges.gitter.im/maxfischer2781/asyncstdlib.svg
+   :alt: Join the chat at https://gitter.im/maxfischer2781/asyncstdlib
+   :target: https://gitter.im/maxfischer2781/asyncstdlib?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge