docs: clarify templating language in use (#2099)

Author: wyardley

CONTRIBUTING.md

@@ -21,6 +21,8 @@ must be refreshed if the module interfaces are changed.
 
 To more cleanly handle cases where desired functionality would require complex duplication of Terraform resources (i.e. [PR 51](https://github.com/terraform-google-modules/terraform-google-kubernetes-engine/pull/51)), this repository is largely generated from the [`autogen`](/autogen) directory.
 
+This uses [Jinja2](https://jinja.palletsprojects.com/en/3.1.x/) under the hood for templating.
+
 The root module is generated by running `make build`. Changes to this repository should be made in the [`autogen`](/autogen) directory where appropriate.
 
 Note: The correct sequence to update the repo using autogen functionality is to run