ci: ask users to provide towncrier snippet

If the release-notes entry isn't required, tag the PR with
'no-release-notes' label and re-run the wokflow.

While on it, provide a TL;DR documentation to simplify the steps for the
first time contributors.

.github/workflows/towncrier.yml

@@ -0,0 +1,55 @@
+---
+name: Check that ChangeLog has been modified
+
+on:
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  changed_files:
+    runs-on: ubuntu-latest
+    name: Test that towncrier file exists
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Get changed files
+        id: changed-files
+        uses: tj-actions/changed-files@v42
+        with:
+          since_last_remote_commit: true
+
+      - name: Detect the missing Release Notes entry
+        env:
+          ALL_CHANGED_FILES: ${{ steps.changed-files.outputs.all_changed_files }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          OWNER: ${{ github.repository_owner }}
+          REPO_NAME: ${{ github.event.repository.name }}
+          PULL_REQUEST_NUMBER: ${{ github.event.pull_request.number }}
+        run: |
+          labels="$(gh api repos/$OWNER/$REPO_NAME/pulls/$PULL_REQUEST_NUMBER --jq '.labels.[].name')"
+          status=false
+
+          case $labels in
+            *no-release-notes*)
+               echo "No release notes entry expected."
+               exit 0
+               ;;
+          esac
+
+          for file in ${ALL_CHANGED_FILES}; do
+            case $file in
+            *release-notes-next*)
+              status=true
+              ;;
+            esac
+          done
+          if $status; then
+            echo "Thank you for providing the release note snippet!"
+          else
+            echo "Please provide release notes snippet, per the instructions"
+            echo https://rpm-software-management.github.io/mock/Release-Notes-New-Entry
+          fi
+          $status

docs/Release-Notes-New-Entry.md

@@ -3,11 +3,24 @@ layout: default
 title: Maintaining ChangeLog
 ---
 
+# TL;DR - quick start
+
+You typically want to create a markdown snippet with:
+
+    $ towncrier create failed-selinux-mountpoint.bugfix
+    Created news fragment at ./releng/release-notes-next/failed-selinux-mountpoint.bugfix
+    $ vim ./releng/release-notes-next/failed-selinux-mountpoint.bugfix
+    ... document ...
+    $ git add ./releng/release-notes-next/failed-selinux-mountpoint.bugfix
+
+Please refer to issues, PRs, bugs, commits using the
+`[reference#ID][]` or `[some text][reference#ID]` syntax described below.
+
 # Maintaining ChangeLog
 
 Mock uses the [towncrier](https://github.com/twisted/towncrier) project for
-maintaining release notes (aka changelog).  For adding new entry, provide a text
-file in
+maintaining release notes (aka changelog).  For adding a new Release Notes
+entry, provide a markdown file in the
 [releng/release-notes-next](https://github.com/rpm-software-management/mock/tree/main/releng/release-notes-next)
 drop-in directory.