doc: Document oneliner to create Git Tag

Signed-off-by: Manuel Hutter <manuel@hutter.io>

README.md

@@ -41,6 +41,14 @@ To publish a new release, simply create & push a new Git Tag.
 
 NOTE: Tags should follow the included CRS version. Use the build number to signify changes to the images with the same CRS number, e.g. `v4.3.0-0` -> `v4.3.0-1`.
 
+One-Liner to create a Tag:
+
+```sh
+tag="v$(grep '^FROM ' Dockerfile| cut -d':' -f2 | cut -d'-' -f1)-0"; git tag -s "$tag" -m "$tag"
+```
+
+Don't forget to `git push --tags` afterwards!
+
 ## Configuration
 
 Most aspects can be configured using environment variables.