Mention Hydrgeon in self-building.md

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1055

docs/self-building.md

@@ -14,6 +14,7 @@ List of roles where self-building the Docker image is currently possible:
 - `matrix-synapse`
 - `matrix-synapse-admin`
 - `matrix-client-element`
+- `matrix-client-hydrogen`
 - `matrix-registration`
 - `matrix-coturn`
 - `matrix-corporal`