Update README.md (#100)

Author: jess

README.md

@@ -224,6 +224,41 @@ pin_all_from File.expand_path("../app/assets/javascripts", __dir__)
 ```
 
 
+## Selectively importing modules
+
+You can selectively import your javascript modules on specific pages.
+
+Create your javascript in `app/javascript`:
+
+```js
+// /app/javascript/checkout.js
+// some checkout specific js
+```
+
+Pin your js file:
+
+```rb
+# config/importmap.rb
+# ... other pins...
+pin "checkout"
+```
+
+Import your module on the specific page. Note: you'll likely want to use a `content_for` block on the specifc page/partial, then yield it in your layout.
+
+```erb
+<% content_for :head do %>
+  <%= javascript_import_module_tag "checkout" %>
+<% end %>
+```
+
+**Important**: The `javacript_import_module_tag` should come after your `javascript_importmap_tags`
+
+```erb
+<%= javascript_importmap_tags %>
+<%= yield(:head) %>
+```
+
+
 ## Include a digest of the import map in your ETag
 
 If you're using [ETags](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag) generated by Rails helpers like `stale?` or `fresh_when`, you need to include the digest of the import map into this calculation. Otherwise your application will return 302 cache responses even when your JavaScript assets have changed. You can avoid this with something like: