Add links to discord server

.github/contributing.md

@@ -15,7 +15,8 @@ in a pull request.
 ## Questions
 Are you just using the software and have a question or improvement?
 
-* Ask it on the [gitter channel](https://gitter.im/wger-project/wger)
+* Ask it on the [gitter channel](https://gitter.im/wger-project/wger),
+* the [discord server](https://discord.gg/rPWFv6W)
 * or just [open an issue](https://github.com/wger-project/wger/issues)
 
 ## Issues

README.md

@@ -104,6 +104,7 @@ didn't behave as you expected. We can't fix what we don't know about, so please
 report liberally. If you're not sure if something is a bug or not, feel free to
 file a bug anyway.
 
+* **discord:** <https://discord.gg/rPWFv6W>
 * **gitter:** <https://gitter.im/wger-project/wger>
 * **issue tracker:** <https://github.com/wger-project/wger/issues>
 * **twitter:** <https://twitter.com/wger_project>

docs/index.rst

@@ -58,11 +58,10 @@ Feel free to contact us if you found this useful or if there was something that
 didn't behave as you expected (in this case you can also open a ticket on the
 issue tracker).
 
+* **discord:** https://discord.gg/rPWFv6W
 * **gitter:** https://gitter.im/wger-project/wger
 * **issue tracker:** https://github.com/wger-project/wger/issues
 * **twitter:** https://twitter.com/wger_project
-* **mailing list:** https://groups.google.com/group/wger / wger@googlegroups.com,
-  no registration needed
 
 
 Sources

extras/docker/apache/README.md

@@ -46,10 +46,10 @@ didn't behave as you expected. We can't fix what we don't know about, so please
 report liberally. If you're not sure if something is a bug or not, feel free to
 file a bug anyway.
 
+* discord: <https://discord.gg/rPWFv6W>
 * gitter: <https://gitter.im/wger-project/wger>
 * issue tracker: <https://github.com/wger-project/wger/issues>
 * twitter: <https://twitter.com/wger_project>
-* mailing list: <https://groups.google.com/group/wger> / wger@googlegroups.com, no registration needed
 
 Sources
 -------

extras/docker/base/README.md

@@ -14,10 +14,10 @@ didn't behave as you expected. We can't fix what we don't know about, so please
 report liberally. If you're not sure if something is a bug or not, feel free to
 file a bug anyway.
 
+* discord: <https://discord.gg/rPWFv6W>
 * gitter: <https://gitter.im/wger-project/wger>
 * issue tracker: <https://github.com/wger-project/wger/issues>
 * twitter: <https://twitter.com/wger_project>
-* mailing list: <https://groups.google.com/group/wger> / wger@googlegroups.com, no registration needed
 
 Sources
 -------

extras/docker/compose/README.md

@@ -66,10 +66,10 @@ didn't behave as you expected. We can't fix what we don't know about, so please
 report liberally. If you're not sure if something is a bug or not, feel free to
 file a bug anyway.
 
+* discord: <https://discord.gg/rPWFv6W>
 * gitter: <https://gitter.im/wger-project/wger>
 * issue tracker: <https://github.com/wger-project/wger/issues>
 * twitter: <https://twitter.com/wger_project>
-* mailing list: <https://groups.google.com/group/wger> / wger@googlegroups.com, no registration needed
 
 ## Sources
 

extras/docker/development/README.md

@@ -65,10 +65,10 @@ didn't behave as you expected. We can't fix what we don't know about, so please
 report liberally. If you're not sure if something is a bug or not, feel free to
 file a bug anyway.
 
+* discord: <https://discord.gg/rPWFv6W>
 * gitter: <https://gitter.im/wger-project/wger>
 * issue tracker: <https://github.com/wger-project/wger/issues>
 * twitter: <https://twitter.com/wger_project>
-* mailing list: <https://groups.google.com/group/wger> / wger@googlegroups.com, no registration needed
 
 ## Sources