docs: document the rds-databases module

terraform/rds-databases/README.md

@@ -0,0 +1,21 @@
+# RDS databases
+
+This module creates a `shared` RDS instance that contains three databases:
+
+- `discord-mods-bot`
+- `triagebot`
+- `rustc_perf`
+
+You can access the DB instance using [bastion].
+
+To run `terraform` commands, use the following command
+to port-forward the bastion host:
+
+```sh
+ssh -L localhost:57467:shared.<id>.us-west-1.rds.amazonaws.com:5432 bastion.infra.rust-lang.org
+```
+
+You can find the full endpoint (including `<id>`) in the
+AWS console.
+
+[bastion]: https://github.com/rust-lang/infra-team/tree/master/service-catalog/bastion