release: officially adopt Keep a Changelog

We are already using the format so might as well link to where it comes
from.

Also adjust the release-notes.sh line to account for 5 additional lines
at the top.

Signed-off-by: Pablo Barbáchano <pablob@amazon.com>

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
 ## [Unreleased]
 
 ### Added

tools/release-notes.sh

@@ -26,7 +26,8 @@ changelog="$FC_ROOT_DIR/CHANGELOG.md"
 
 # Patterns for the sections in the changelog corresponding to the versions.
 pat_curr="/^##\s\[$curr_ver\]/"
-pat_prev="/^##\s\[.+]/ && NR>3"
+# Match the previous version, skipping the 8 lines at the top.
+pat_prev="/^##\s\[.+]/ && NR>8"
 # Extract the section enclosed between the 2 headers and strip off the first
 # 2 and last 2 lines (one is blank and one contains the header `## [A.B.C]`).
 # Then, replace `-` with `*` and remove section headers.