Add a Gitter chat badge to README.md

[gitter-badger]

johanbrandhorst/certify now has a Chat Room on Gitter

@johanbrandhorst has just created a chat room. You can visit it here: https://gitter.im/go-certify/community.

This pull-request adds this badge to your README.md:

If my aim is a little off, please let me know.

Happy chatting.

PS: Click here if you would prefer not to receive automatic pull-requests from Gitter in future.

[codecov]

Codecov Report

Merging #66 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #66   +/-   ##
=======================================
  Coverage   67.91%   67.91%           
=======================================
  Files           8        8           
  Lines         402      402           
=======================================
  Hits          273      273           
  Misses         81       81           
  Partials       48       48

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