Add pointers to matrix-chat (#139)

Author: msirringhaus

CONTRIBUTING.md

@@ -47,6 +47,10 @@ And then there is, of course, writing code!
 
 Here is a list of helpful resources you can consult:
 
+## Discussion platform
+
+Join the discussion on Matrix at `#credentials-for-linux:matrix.org`.
+
 ## Documentation
 
 To help you get started, we have provided documentation for various parts of the

README.md

@@ -57,6 +57,12 @@ with `credentialsd` directly without an add-on. You can access a
 
 [firefox-patch-flatpak]: https://download.opensuse.org/repositories/home:/MSirringhaus:/webauthn_devel/openSUSE_Factory_flatpak/
 
+## Contributing
+
+We welcome contributions! See [CONTRIBUTING.md](/CONTRIBUTING.md) for details.
+
+Join the discussion on Matrix at `#credentials-for-linux:matrix.org`.
+
 ## Mockups
 
 Here are some mockups of what this would look like for a user: