doc(CONTRIBUTING.md): add

Signed-off-by: Lars Wirzenius <liw@liw.fi>

CONTRIBUTING.md

@@ -0,0 +1,27 @@
+# Contributing to the Radicle CI broker
+
+The Radicle project welcomes contributions to the CI broker, as to all
+other components of Radicle.
+
+Development happens via Radicle. To propose a change, make a patch. If
+you suspect the change needs discussion first, please first open an
+issue or a topic on Zulip (preferably in the `#radicle-ci` channel).
+
+* See [`README.md`](README.md) for how to run the test suite.
+* We try to use [conventional
+  commits](https://www.conventionalcommits.org/en/v1.0.0/), but it's
+  not required.
+* We use [Subplot](https://subplot.tech/) for documenting acceptance
+  criteria and verifying they are met. This also acts as an
+  integration test suite. We also test upgrades in the subplot.
+* Unit tests are done in the usual Rust way.
+* We use Radicle CI to test changes: we run [an
+  instance](https://ci0.liw.fi/zwTxygwuz5LDGBq255RA2CbNGrz8.html) and
+  a patch needs to pass CI to be merged. See
+  [`.radicle/ambient.yaml`](.radicle/ambient.yaml).
+
+If you're unsure about anything, don't hesitate to ask.
+
+There are occasional office hours, where anything related to Radicle
+CI can be discussed. Check the `#radicle-ci` topic on Zulip for when
+and where.