build: add new GH actions to require release note updates

In this commit, we add a simple bash script to parse out the current PR
number from an environment variable in the GH actions context, and use
that to check to see if the PR has been referenced in the release notes
or not. This isn't 100% fool proof, but it should catch most of the
common cases.

scripts/check-release-notes.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+
+set -e
+
+# Extract the PR number which is stored in the $GITHUB_REF env variable. The
+# format of the string stored in the variable is: refs/pull/:PRNUMBER/merge.
+PR_NUMBER=$(echo $GITHUB_REF | awk 'BEGIN { FS = "/" } ; { print $3 }')
+
+# Ensure that the PR number at least shows up in the release notes folder under
+# one of the contained milestones.
+if ! grep -r -q "lightningnetwork/lnd/pull/$PR_NUMBER" docs/release-notes; then
+    echo "PR $PR_NUMBER didn't update release notes"
+    exit 1
+fi