gitlab-ci: document the get_version implicit nature

Had a silly moment a while back, assuming the stage is no longer needed.
That's not the case, so add a brief commit about that.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

.gitlab-ci.yml

@@ -37,6 +37,8 @@ lint:
     - releases
     - tags
 
+# This is an implicit gitlab stage, with the build.env variables used by either
+# other stages or auxiliarry scripts.
 get_version:
   stage: .pre
   script: