debian/README.source: add new file

Signed-off-by: Héctor Orón Martínez <hector.oron@collabora.com>

debian/README.source

+Useful git remotes
+==================
+
+git remote add salsa https://salsa.debian.org/ruby-team/open-build-service.git
+git remote add upstream https://github.com/openSUSE/open-build-service/