Add contributors guidelines to template

Is Your Feature Request Related to a Problem? Please Describe

Currently the package template does not include a CONTRIBUTING.md guidelines page, but this is quite a well suited page for templating given a basic guide can use largely pro-forma text with a few templated links. While it will need to be relatively generic, I think having something basic included by default in template would be good.

Describe the Solution You'd Like

A CONTRIBUTING.md contributors guide in the cookicutter template. We added a basic guide to S2FFT recently which could be used as starting point (as could probably many others this one is just recent in memory!).

Describe Alternatives You've Considered

No response

Additional Context

No response

Start by locating the cookiecutter package template and reviewing the linked S2FFT CONTRIBUTING.md guide as a possible model. Add a generic CONTRIBUTING.md with templated links, then verify that newly generated Python packages include the guide and that its links are appropriate for the template.