Clear release notes (#205)

Restores the release notes template after
[v1.5.0](https://github.com/frequenz-floss/frequenz-core-python/releases/tag/v1.5.0),
as required by the contributing guide.

Author: llucax

RELEASE_NOTES.md

@@ -1,11 +1,17 @@
 # Frequenz Core Library Release Notes
 
-## New Features
+## Summary
+
+<!-- Here goes a general summary of what this release is about -->
+
+## Upgrading
 
-A new `frequenz.core.warnings` module with:
+<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
+
+## New Features
 
-- `ignoring_warnings()` and `ignoring_deprecations()` to silence warnings/deprecations around a piece of code. Unlike `warnings.catch_warnings()`, entering and leaving it doesn't reset the warnings deduplication history of the program, so warnings already shown are not shown again, working around [python/cpython#73858](https://github.com/python/cpython/issues/73858). It costs about the same as the standard library block, so it is also usable on a hot path, where repairing the damage afterwards would not be.
+<!-- Here goes the main new features and examples or instructions on how to use them -->
 
-- `asserting_no_warnings()`, and its `asserting_no_deprecations()` shortcut, fail when the code in the block raises a matching warning/deprecation, listing each one with the place it came from. They are meant for tests, and are a better tool than an `"error"` filter, which makes `warnings.warn()` raise inside the code under test and so changes the very behaviour the test is checking.
+## Bug Fixes
 
-- `deprecated_aliases()` builds a module `__getattr__` that warns when a symbol that moved to another module, or was renamed, is reached through its old import path, serving the very same object so `isinstance` keeps working through both paths.
+<!-- Here goes notable bug fixes that are worth a special mention or explanation -->