docs: add reference to update_init_branches.sh (#7)

# Description

Add a `.github/update_init_branches.sh` to update the init branches
automatically.

# Verification

Script manually executed.

.github/update_init_branches.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+#
+# This script rebases all init branches on the main branch.
+#
+
+git checkout main
+git pull
+
+# for all init branches
+for branch in $(git branch --list "init-*"); do
+  git checkout "$branch"
+  git merge main
+  git push
+done

README.md

@@ -4,20 +4,26 @@ Template repository for Maven projects. Don't forget to update the README.md fil
 run
 
 ```shell
-.github/update_workflows.sh
+git checkout main
 .github/update_templates.sh
 
 # strongly suggested!
 pre-commit install -c .github/pre-commit-config.yaml
 ```
 
+In case a major release is published on the Workflow-Template repository, run `.github/update_workflows.sh`. This creates a PR
+with the changes in the workflows. Carefully check the changes and merge them into the `main` branch.
+
 ## Development
 
 ### Usage
 
 This repository is used as a template repository for new Maven projects. Thus the newly created repository will have the same
 structure and configurations as this one.
 
+After merging into the `main` branch, use `.github/update_init_branches.sh` to merge these changes into the `init-` branches as
+well.
+
 ### `init-` branches
 
 The `init-` branches are used to initialize the project with the necessary files and configurations. Create them in this repository