Update CONTRIBUTING.md

Reminder for schema.yaml updates

CONTRIBUTING.md

@@ -14,6 +14,7 @@ Before submitting the PR review that you have included the following changes, wh
 - Helm Chart: If your new feature introduces settings consider adding those to the Helm chart [README.md](chart/elastalert2/README.md) and [values.yaml](chart/elastalert2/values.yaml)
 - Examples: If your new feature includes new configuration options, review the [Example config file](examples/config.yaml.example) to see if they should be added there for consistency with other configuration options.
 - Change log: Describe your contribution to the appropriate section(s) for the _Upcoming release_, in the [CHANGELOG.md](CHANGELOG.md) file.
+- Schema: Update the schema.yaml file with new parameters for existing and new alerters and rule types.
 
 ## Releases