Update release notes and prepare them for v0.18.4

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

Author: llucax

RELEASE_NOTES.md

@@ -1,17 +1,8 @@
 # Frequenz Microgrid API Client 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
+* `Component` classes now expose two new functions to check the operational mode:
 
-<!-- Here goes notable bug fixes that are worth a special mention or explanation -->
+    - `provides_telemetry()`: whether the component provides telemetry data
+    - `accepts_control()`: whether the component accepts control commands