doc(README): Replace Gitter badge with Discourse
doc(README): Replace Gitter badge with Discourse

This replaces the Gitter.im badge with a Discourse badge pointing
to the Etcher topics in the Resin.io Forums.

Change-Type: patch

resin-io/etcher,resin-io/herostratus,resin-io/etcher,resin-io/herostratus,resin-io/etcher,resin-io/resin-etcher,resin-io/etcher,resin-io/resin-etcher,resin-io/etcher