Improve github action build-pdf

- Set better git user.name & user.email fields
- Set better git commit message
- Add paths-ignore push filter

.github/workflows/build-pdf.yml

@@ -3,6 +3,10 @@ on:
   push:
     branches:
       - 'master'
+    paths-ignore:
+      - 'README.md'
+      - 'Makefile'
+      - 'LICENSE.txt'
   workflow_dispatch:
 
 jobs:
@@ -44,10 +48,10 @@ jobs:
       # Commit mmbtools.pdf
       - name: commit-pdf
         run: |
-          git config user.name github-actions
-          git config user.email github-actions@github.com
+          git config user.name "Open Digital Radio"
+          git config user.email github@opendigitalradio.org
           git add -f mmbtools.pdf
-          git commit -m "generated"
+          git commit -m "generated by github action build-pdf"
         working-directory: gh-pages
 
       - name: push-pdf