add creation timestamp to README

Couldn't get these [instructions](https://cookiecutter.readthedocs.io/en/stable/advanced/injecting_context.html) to work.

```
The `{{ cookiecutter.package_name }}` Python package was [created from template](https://develop.qiime2.org/en/latest/plugins/tutorials/create-from-template.html) on {{ cookiecutter.timestamp }}.
```