Add information about heroku to readme

README.md

@@ -19,5 +19,12 @@ Also ensure that your `Rails.application.config.asset_host` is setup correctly s
 The gem then uploads the sourcemaps after the `assets:precompile` rake task. 
 It searches for all `.map` files within the `Rails.public_path` directory.
 
+### Heroku
+When building on heroku, make sure you set the revision based on the `SOURCE_VERSION` environment variable. This variable is set during the build phase and will be equal to the `HEROKU_SLUG_COMMIT` revision on runtime. Note that you should have [heroku dyno metadata](https://devcenter.heroku.com/articles/dyno-metadata) enabled.
+```yaml
+default:
+  revision: "<%= ENV['SOURCE_VERSION'] || ENV.fetch('HEROKU_SLUG_COMMIT', 'unknown') %>"
+```
+
 ## Publishing a new version
 The [publish workflow](.github/workflows/publish.yml) listens to a new release in GitHub.