add a template to add missing tokenization test (#16553)
* add a template to add missing tokenization test
* add cookiecutter setting
* improve doc
* Update templates/adding_a_missing_tokenization_test/README.md
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>