Add release notes

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

RELEASE_NOTES.md

@@ -6,19 +6,27 @@
 
 ## Upgrading
 
-<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
+- `flake8` basic checks are enabled now. Most are already covered by `pylint`, but there are a few differences, so you might need to fix your code if `flake8` find some issues.
+
+- `darglint` was replaced by `pydoclint`, `pydoclint` can find a few more issues than `darglint`, so your code might need adjusting.
+
+- `darglint` is not used anymore, but if it is installed, it will make `flake8` run extremely slowly anyways, so it is extremely recommended to uninstall it (`pip uninstall darglint`) and rebuild you `nox` *venvs* if you use `-R`.
+
+- If you are upgrading without regenerating the cookiecutter templates, you'll need to adjust the dependencies accordingly.
 
 ### Cookiecutter template
 
-<!-- Here upgrade steps for cookiecutter specifically -->
+- See the general upgrading section.
 
 ## New Features
 
-<!-- Here goes the main new features and examples or instructions on how to use them -->
+- `flake8` is now used to check the files.
+
+- `darlint` was replaced by `pydoclint`, which is way faster and detect more issues.
 
 ### Cookiecutter template
 
-<!-- Here new features for cookiecutter specifically -->
+- See the general new features section.
 
 ## Bug Fixes