docs: Start some matrix docs

docs/matrix.md

+# Matrix
+
+We offer a Matrix homeserver for Arch team members. Matrix is a federated communication service with
+a variety of available clients for multiple platforms, mobile included. The flagship [Element
+clients](https://element.io/) offer us file upload, end-to-end encryption, push notifications and
+integrations with third-party services.
+
+## Signing in
+
+For the initial sign-in you need to use a client that supports OpenID Single-Sign-On, such as [Element
+Web](https://app.element.io/). Enter `@username:archlinux.org` as the username and Element should
+offer to sign into our homeserver.
+
+You will be automatically invited to several rooms:
+  - `#archlinux:archlinux.org`: A public room for Arch Linux users.
+  - `#staff:archlinux.org`: Bridged with the `#archlinux-staff` IRC channel on Freenode.
+  - `#internal:archlinux.org`: A staff-only room with end-to-end encryption.
+
+After signing in you can use Element's settings to set a password for the account if you want to use
+a client that does not support SSO.
+
+If you need to provide your client with a homeserver address, use `https://matrix.archlinux.org`.
+
+## IRC bridges
+
+### Our bridge
+
+We bridge several of our private IRC channels on Freenode to Matrix, which you need to be invited
+into:
+  - `#developers:archlinux.org`: Bridged with `#archlinux-dev`.
+  - `#trusted-users:archlinux.org`: Bridged with `#archlinux-tu`.
+  - `#staff:archlinux.org`: Bridged with `#archlinux-staff`.
+
+### Matrix.org bridge
+
+Channels without keys are available via the "official" Freenode bridge at Matrix.org. For example:
+  - `#freenode_#archlinux-devops:matrix.org`: Bridged with `#archlinux-devops`.
+  - `#freenode_#archlinux-projects:matrix.org`: Bridged with `#archlinux-projects`.
+
+**Please avoid joining large bridged rooms (such as `#freenode_#archlinux:matrix.org`), as these
+slow down the server immensely.**
+
+Freenode may require you to have a registered nick to join certain channels. Once
+`@appservice-irc:matrix.org` contacts you, tell it to `!storepass <username>:<password>` with the
+username and the password of your Freenode account and it will reconnect you as registered.