Add matrix-mx-puppet-skype to the list of roles that can self-build

docs/self-building.md

@@ -8,12 +8,13 @@ matrix_container_images_self_build: true
 ```
 Setting that variable will self-build every role which supports self-building. Self-building can be set on a per-role basis as well.
 
-List of roles where self-building the docker image is currently possible:
-- synapse
-- riot-web
-- coturn
-- ma1sd
-- matrix-bridge-mautrix-facebook
-- matrix-bridge-mautrix-hangouts
+List of roles where self-building the Docker image is currently possible:
+- `matrix-synapse`
+- `matrix-riot-web`
+- `matrix-coturn`
+- `matrix-ma1sd`
+- `matrix-mautrix-facebook`
+- `matrix-mautrix-hangouts`
+- `matrix-mx-puppet-skype`
 
 Adding self-building support to other roles is welcome. Feel free to contribute!