docs: add AGit pull request instructions

CONTRIBUTING.md

@@ -0,0 +1,15 @@
+# Contributing
+
+## Opening a pull request on git.zacharyzhang.com
+
+Forking is disabled on this server. Open pull requests with AGit instead:
+
+1. Register an account at https://git.zacharyzhang.com
+2. Clone, commit your changes, then push to a `refs/for/<target-branch>` ref:
+
+   ```bash
+   git clone https://git.zacharyzhang.com/ZacharyZhang-NY/AgentCompany.git
+   git push origin HEAD:refs/for/main -o topic=<short-name> -o title="<PR title>" -o description="<PR body>"
+   ```
+
+3. To update the pull request, push again with the same `topic`.