README: Document custom OBS/repositories

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>

README.md

@@ -28,3 +28,15 @@ Build platform-specific image for Raspberry Pi using a previously built ospack
 ```
 debos --scratchsize=10G image-rpi64.yaml
 ```
+
+## Custom OBS/repositories
+
+The custom images recipes might pull certain packages from extra repositories
+made specifically to this purpose.
+
+### bliss:v2021:docker
+
+Packages backported to Apertis v2021 for Raspberry Pi:
+
+* OBS instance: https://build.collabora.co.uk/project/show/bliss:v2021:docker
+* Apertis repository: https://repositories.apertis.org/bliss/dists/v2021/docker/