Docs & readme - remove references to now closed developers forum

- remove references to https://groups.google.com/forum/#!forum/wagtail-developers
- add references to the Wagtail slack workspace

README.md

@@ -92,7 +92,7 @@ To try out the latest features before a release, we also create builds from mast
 
 ### Contributing
 
-If you're a Python or Django developer, fork the repo and get stuck in! We run a separate group for developers of Wagtail itself at https://groups.google.com/forum/#!forum/wagtail-developers (please note that this is not for support requests).
+If you're a Python or Django developer, fork the repo and get stuck in! We have several developer focused channels on the [Slack workspace](https://github.com/wagtail/wagtail/wiki/Slack).
 
 You might like to start by reviewing the [contributing guidelines](https://docs.wagtail.io/en/latest/contributing/index.html) and checking issues with the [good first issue](https://github.com/wagtail/wagtail/labels/good%20first%20issue) label.
 

docs/contributing/developing.rst

@@ -7,7 +7,7 @@ Setting up a local copy of `the Wagtail git repository <https://github.com/wagta
 
 If you're happy to develop on a virtual machine, the `vagrant-wagtail-develop <https://github.com/wagtail/vagrant-wagtail-develop>`_ and `docker-wagtail-develop <https://github.com/wagtail/docker-wagtail-develop>`_ setup scripts are the fastest way to get up and running. They will provide you with a running instance of the `Wagtail Bakery demo site <https://github.com/wagtail/bakerydemo/>`_, with the Wagtail and bakerydemo codebases available as shared folders for editing on your host machine.
 
-(Build scripts for other platforms would be very much welcomed - if you create one, please let us know via the `Wagtail Developers group <https://groups.google.com/forum/#!forum/wagtail-developers>`_!)
+(Build scripts for other platforms would be very much welcomed - if you create one, please let us know via the `Slack workspace <https://github.com/wagtail/wagtail/wiki/Slack>`_!)
 
 If you'd prefer to set up all the components manually, read on. These instructions assume that you're familiar with using pip and virtualenv to manage Python packages.