[VVP-4620] Create RELEASING.md file

RELEASING.md

@@ -0,0 +1,15 @@
+# Releasing
+
+## Branches and Tags
+
+Branch/Tag | Description
+---|---
+`master` branch | Code for the *next* release.
+`release-X.Y` branch | Code for the *vX.Y* release series.
+
+### Creating a release branch
+
+We have to create a new release branch manually from the `master` branch.
+
+When a new release branch was created, some configuration files have to be adapted on `master`:
+- Add a [mergify rule](.mergify.yml) for backporting to the previous version, analogous to the existing ones.