Add documentation for mirroring to GitHub

docs/github-mirror.md

+# How to add a mirror to GitHub
+
+If you want to mirror your repository "myproject" from gitlab.archlinux.org to the github.com/archlinux organization,
+you should create an empty project for your project at github.com/archlinux/myproject or
+if that's an existing repository, make sure that the current histories of the source and
+target repository are exactly the same.
+
+Then, go to https://gitlab.archlinux.org/archlinux/myproject/-/settings/repository and open
+"Mirroring Repositories".
+Make sure it has these settings:
+
+* Git repository URL: `ssh://git@github.com/archlinux/myproject.git`
+* Mirror direction: Push
+* Authentication method: "SSH public key"
+* Only mirror protected branches: Off
+
+and then click Mirror repository.
+
+A new entry will pop up which has a button titled "Copy SSH public key". Click that.
+
+Then go to https://github.com/archlinux/myproject/settings/keys and add a new deploy key.
+Give it the title "gitlab.archlinux.org" so we know where it's from and then paste the
+public key you copied from GitLab just now.
+
+Your push mirror should now work.