update theme-template README (#6257)

Merge pull request 6257

lib/theme_template/README.md.erb

@@ -1,9 +1,12 @@
 # <%= theme_name %>
 
-Welcome to your new Jekyll theme! In this directory, you'll find the files you need to be able to package up your theme into a gem. Put your layouts in `_layouts`, your includes in `_includes` and your sass in `_sass`. To experiment with this code, add some sample content and run `bundle exec jekyll serve` – this directory is setup just like a Jekyll site!
+Welcome to your new Jekyll theme! In this directory, you'll find the files you need to be able to package up your theme into a gem. Put your layouts in `_layouts`, your includes in `_includes`, your sass files in `_sass` and any other assets in `assets`.
+
+To experiment with this code, add some sample content and run `bundle exec jekyll serve` – this directory is setup just like a Jekyll site!
 
 TODO: Delete this and the text above, and describe your gem
 
+
 ## Installation
 
 Add this line to your Jekyll site's `Gemfile`:
@@ -28,7 +31,7 @@ Or install it yourself as:
 
 ## Usage
 
-TODO: Write usage instructions here. Describe your available layouts, includes, and/or sass.
+TODO: Write usage instructions here. Describe your available layouts, includes, sass and/or assets.
 
 ## Contributing
 
@@ -40,7 +43,8 @@ To set up your environment to develop this theme, run `bundle install`.
 
 Your theme is setup just like a normal Jekyll site! To test your theme, run `bundle exec jekyll serve` and open your browser at `http://localhost:4000`. This starts a Jekyll server using your theme. Add pages, documents, data, etc. like normal to test your theme's contents. As you make modifications to your theme and to your content, your site will regenerate and you should see the changes in the browser after a refresh, just like normal.
 
-When your theme is released, only the files in `_layouts`, `_includes`, and `_sass` tracked with Git will be released.
+When your theme is released, only the files in `_layouts`, `_includes`, `_sass` and `assets` tracked with Git will be bundled.
+To add a custom directory to your theme-gem, please edit the regexp in `<%= theme_name %>.gemspec` accordingly.
 
 ## License