docs: added docs on mattermost users and calls (#278)

* Added docs to save others time

* polished docs

* Moved mattermost sso notes to development notes

* moved to the existing development doc and re-ordered paragraphs

docs/development.md

@@ -77,3 +77,24 @@ destroy-ns <namespace>
 # So we don't dirty the git history
 rm temp.json
 ```
+
+## Mattermost
+
+### SSO
+
+If creating totally new users, it's best to have them use the sign-up process in Keycloak to ensure properly configured users. Importing users from
+an external system en-masse into the UDS Keycloak is outside the scope of this document.
+
+When creating users for Mattermost manually in Keycloak, be sure to put a `mattermostid` in for the user. If you don't, when you sign into Mattermost `via GitLab` you'll get the `Could not parse auth data out of gitlab user object` error.
+
+![image](screenshots/mattermost-sso-missing-mattermostid.png)
+
+If you create an invalid `mattermostid` (such as entering a person's name) you'll get an invalid OAuth object error. It expects a (unique) string of numbers.
+
+### Enabling Calls
+
+The calls plugin starts out in [test mode](https://docs.mattermost.com/configure/plugins-configuration-settings.html#test-mode). Administrators must turn it on if it's desired.
+
+![image](screenshots/enable-call-plugin.png)
+
+This is presently a known issue (calls don't work). Review [#73](https://github.com/defenseunicorns/uds-bundle-software-factory-nutanix/issues/73) for updates.