Add a Gitter chat badge to README.rst

[gitter-badger]

BlueBrain/EModelRunner now has a Chat Room on Gitter

@wvangeit has just created a chat room. You can visit it here: https://gitter.im/BlueBrain/EmodelRunner.

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

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 #24 (d80540e) into main (295163c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #24   +/-   ##
=======================================
  Coverage   64.54%   64.54%           
=======================================
  Files          43       43           
  Lines        3404     3404           
=======================================
  Hits         2197     2197           
  Misses       1207     1207           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 295163c...d80540e. Read the comment docs.