Mention Igniter in v0.26 changelog migration guide (#397)

Co-authored-by: Andrea Leopardi <an.leopardi@gmail.com>

Author: maennchen

CHANGELOG.md

@@ -48,6 +48,12 @@ We are also updating [Phoenix](https://github.com/phoenixframework/phoenix) gene
 
 If you update Gettext and still use `use Gettext, otp_app: :my_app` to define a backend, Gettext will emit a warning now.
 
+### Migration with Igniter
+
+If your project is using [`igniter`](https://hex.pm/packages/igniter), you can run
+[`mix igniter.update_gettext`](https://hexdocs.pm/igniter/Mix.Tasks.Igniter.UpdateGettext.html)
+to automatically migrate to the new API.
+
 ### Detailed Changelog
 
 This is a detailed list of the new things introduced in this release: