add documentation for push-extra-tags option

docs/advanced-usage/registry-management.md

@@ -135,3 +135,34 @@ dokku registry:set node-js-app push-on-release
 # globally
 dokku registry:set --global push-on-release
 ```
+
+### Push extra tags 
+
+To push the image on release with extra tags, set the `push-extra-tags` to a comma-separated list of tags via the `registry:set` command. The default value for this property is `false`. Setting the property will result in the image being tagged with extra tags every release.
+
+```shell
+# multiple-tags
+dokku registry:set node-js-app push-extra-tags foo,bar
+```
+The `push-extra-tags` can be set to a single tag too.
+
+```shell
+# single tag
+dokku registry:set node-js-app push-extra-tags foo
+```
+
+This property can be set for a single app or globally via the `--global` flag. When set globally, the app-specific value will always overide the global value. The default global value for this property is `false`.
+
+```shell
+dokku registry:set --global push-extra-tags foo,bar
+```
+
+Setting the property value to an empty string will reset the value to the system default. Resetting the value can be done per app or globally.
+
+```shell
+# per-app
+dokku registry:set node-js-app push-extra-tags
+
+# globally
+dokku registry:set --global push-extra-tags
+```