Clear release notes and migration script

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

RELEASE_NOTES.md

@@ -1,7 +1,37 @@
 # Frequenz Repository Configuration Release Notes
 
+## Summary
+
+<!-- Here goes a general summary of what this release is about -->
+
+## Upgrading
+
+<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
+
+### Cookiecutter template
+
+All upgrading should be done via the migration script or regenerating the templates.
+
+```bash
+curl -sSL https://raw.githubusercontent.com/frequenz-floss/frequenz-repo-config-python/v0.12/cookiecutter/migrate.py | python3
+```
+
+But you might still need to adapt your code:
+
+<!-- Here upgrade steps for cookiecutter specifically -->
+
+## New Features
+
+<!-- Here goes the main new features and examples or instructions on how to use them -->
+
+### Cookiecutter template
+
+<!-- Here new features for cookiecutter specifically -->
+
 ## Bug Fixes
 
+<!-- Here goes notable bug fixes that are worth a special mention or explanation -->
+
 ### Cookiecutter template
 
-- Fixed an outdated link to instructions on how to continue when the cookiecutter template generation finishes.
+<!-- Here bug fixes for cookiecutter specifically -->