Clear release notes (#7)

Author: llucax

RELEASE_NOTES.md

@@ -1,5 +1,17 @@
 # Frequenz Filter Lines Superfence Release Notes
 
-This is the first release of this extension, a superfence for
-[pymdown-extensions](https://github.com/facelessuser/pymdown-extensions) to
-filter code block lines.
+## 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 -->