add missing step for gem-based theme conversion (#6802)

Merge pull request 6802

docs/_docs/themes.md

@@ -3,7 +3,7 @@ title: Themes
 permalink: /docs/themes/
 ---
 
-Jekyll has an extensive theme system that allows you to leverage community-maintained templates and styles to customize your site's presentation. Jekyll themes package up layouts, includes, and stylesheets in a way that can be overridden by your site's content.
+Jekyll has an extensive theme system that allows you to leverage community-maintained templates and styles to customize your site's presentation. Jekyll themes specify plugins and package up assets, layouts, includes, and stylesheets in a way that can be overridden by your site's content.
 
 ## Understanding gem-based themes
 
@@ -108,7 +108,48 @@ Suppose you want to get rid of the gem-based theme and convert it to a regular t
 
 To do this, copy the files from the theme gem's directory into your Jekyll site directory. (For example, copy them to `/myblog` if you created your Jekyll site at `/myblog`. See the previous section for details.)
 
-Then remove references to the theme gem in `Gemfile` and configuration. For example, to remove `minima`:
+Then you must tell Jekyll about the plugins that were referenced by the theme. You can find these plugins in the theme's gemspec file as runtime dependencies. If you were converting the Minima theme, for example, you might see:
+
+```
+spec.add_runtime_dependency "jekyll-feed", "~> 0.9"
+spec.add_runtime_dependency "jekyll-seo-tag", "~> 2.1"
+```
+
+You should include these references in the `Gemfile` in one of two ways.
+
+You could list them individually in both `Gemfile` and `_config.yml`.
+
+```ruby
+# ./Gemfile
+
+gem "jekyll-feed", "~> 0.9"
+gem "jekyll-seo-tag", "~> 2.1"
+```
+
+```yaml
+# ./_config.yml
+
+plugins:
+  - jekyll-feed
+  - jekyll-seo-tag
+```
+
+Or you could list them explicitly as Jekyll plugins in your Gemfile, and not update `_config.yml`, like this:
+
+```ruby
+# ./Gemfile
+
+group :jekyll_plugins do
+  gem "jekyll-feed", "~> 0.9"
+  gem "jekyll-seo-tag", "~> 2.1"
+end
+```
+
+Either way, don't forget to `bundle update`.
+
+However, if you're publishing on GitHub Pages you should update only your `_config.yml` as GitHub Pages doesn't load plugins via Bundler.
+
+Finally, remove references to the theme gem in `Gemfile` and configuration. For example, to remove `minima`:
 
 - Open `Gemfile` and remove `gem "minima", "~> 2.0"`.
 - Open `_config.yml` and remove `theme: minima`.