Add section about canonical rack headers

i18n.md

@@ -119,7 +119,20 @@ RSpec.describe 'Multilanguage support' do
 end
 ```
 
-## HTTP Headers
+## Simple Canonical headers
+
+Add our [`rack-canonical-header` gem](https://github.com/renuo/rack-canonical-header)
+to your `Gemfile` in production to automatically patch the HTTP header tag `Link`.
+
+```rb
+group :production do
+  gem 'rack-canonical-header'
+end
+```
+
+This requires you to provide an env variable `CANONICAL_HOST` in production.
+
+## Advanced headers for multi-language support.
 
 The following sets `Content-Language` and `Link` headers (`canonical` and `hreflang`):