message: Add gitter badge to the readme
  Following and
diff: @@ -8,7 +8,7 @@ Please read the `API documentation here`__
  __ https://docs.rs/petgraph/
  -|build_status|_ |crates|_
  +|build_status|_ |crates|_ |gitter|_
  .. |build_status| image:: https://github.com/petgraph/petgraph/workflows/Continuous%20integration/badge.svg?branch=master
  .. _build_status: https://github.com/petgraph/petgraph/actions
  @@ -16,6 +16,9 @@ __ https://docs.rs/petgraph/
  .. |crates| image:: http://meritbadge.herokuapp.com/petgraph
  .. _crates: https://crates.io/crates/petgraph
  +.. |gitter| image:: https://badges.gitter.im/petgraph-rs/community.svg
  +.. _gitter: https://gitter.im/petgraph-rs/community?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge
  +
  Crate feature flags:
  - ``graphmap`` (default) enable ``GraphMap``.
  