Add gitter badge to README.rst

[jettify]

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[codecov-io]

Codecov Report

Merging #2079 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2079   +/-   ##
=======================================
  Coverage   97.08%   97.08%           
=======================================
  Files          38       38           
  Lines        7689     7689           
  Branches     1344     1344           
=======================================
  Hits         7465     7465           
  Misses        102      102           
  Partials      122      122

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