Add GitHub Pages helpful notes (#9235)

Merge pull request 9235

docs/_docs/step-by-step/10-deployment.md

@@ -42,6 +42,11 @@ bundle exec jekyll serve
 
 This restricts your Ruby environment to only use gems set in your `Gemfile`.
 
+Note: if publishing your site with GitHub Pages, you can match production
+version of Jekyll by using the `github-pages` gem instead of `jekyll`
+in your `Gemfile`. In this scenario you may also want to exclude `Gemfile.lock`
+from your repository because GitHub Pages ignores that file.
+
 ## Plugins
 
 Jekyll plugins allow you to create custom generated content specific to your