Add a Gitter chat badge to README.md

[gitter-badger]

dymaxionlabs/dask-rasterio now has a Chat Room on Gitter

@munshkr has just created a chat room. You can visit it here: https://gitter.im/dymaxionlabs/dask-rasterio.

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

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

Happy chatting.

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[codecov-io]

Codecov Report

Merging #1 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master     #1   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           3      3           
  Lines          67     67           
=====================================
  Hits           67     67

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