Add RELEASE_NOTES to cookiecutter (frequenz-floss#65)

A top-level `RELEASE_NOTES.md` file is added, and a
`.github/RELEASE_NOTES.template.md` template are added, as the template
is already mentioned in the `CONTRIBUTING.md` guide.

Fixes frequenz-floss#9.

Author: llucax

cookiecutter/{{cookiecutter.github_repo_name}}/.github/RELEASE_NOTES.template.md

@@ -0,0 +1,17 @@
+# {{cookiecutter.title}} 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 --> 
+
+## New Features
+
+<!-- Here goes the main new features and examples or instructions on how to use them -->
+
+## Bug Fixes
+
+<!-- Here goes notable bug fixes that are worth a special mention or explanation -->

cookiecutter/{{cookiecutter.github_repo_name}}/RELEASE_NOTES.md

@@ -0,0 +1,17 @@
+# {{cookiecutter.title}} 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 --> 
+
+## New Features
+
+<!-- Here goes the main new features and examples or instructions on how to use them -->
+
+## Bug Fixes
+
+<!-- Here goes notable bug fixes that are worth a special mention or explanation -->