github: add separate README

This README will show up in the repo root on GitHub only. It explains
that the repository is mirrored and directs users to the canonical
Codeberg repository.

Author: mlugg

.github/README.md

@@ -0,0 +1,21 @@
+# setup-zig
+
+Install the Zig compiler for use in a Forgejo Actions or GitHub Actions workflow, and preserve the
+Zig cache across workflow runs.
+
+> [!IMPORTANT]
+>
+> # [This project has moved to Codeberg][main-repo]
+>
+> **Visit the [main repository][main-repo] for documentation and usage examples.**
+>
+> Unfortunately, GitHub Actions includes vendor lock-in mechanisms which require this mirror to exist.
+> It is automatically synchronized with the upstream repository, but issues and pull requests are not
+> accepted here.
+>
+> setup-zig will continue to support GitHub Actions into the future---that isn't changing. However, I
+> strongly encourage users to consider moving to other Git forges such as [Codeberg][codeberg] in
+> order to sidestep the continued enshittification and neglect of GitHub and GitHub Actions.
+
+[main-repo]: https://codeberg.org/mlugg/setup-zig/
+[codeberg]: https://codeberg.org/