add tip to GitHub Actions example about preserving preprocessor condi…

…tionals

README.adoc

@@ -288,6 +288,7 @@ jobs:
     - name: Install Asciidoctor Reducer
       run: sudo gem install asciidoctor-reducer
     - name: Reduce README
+      # to preserve preprocessor conditionals, add the --preserve-conditionals option
       run: asciidoctor-reducer -o README.adoc README-source.adoc
     - name: Commit and Push README
       uses: EndBug/add-and-commit@v9