Add contributors guidelines to template

[matt-graham]

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

[paddyroddy]

Just want to point out that I made a generic one for this repo that was shot down #252. People can be quite opinionated about contributing guidelines.

[matt-graham]

#252 was for contributing guidelines for this repository rather than a contributor guideline to include within 🍪 template right? I can see that what exactly should appear in the contributor guidelines could be a controversial topic but I think it should be possible to get something basic that we all agree on as a common starting point.

[samcunliffe]

How about a pro-forma that isn't actually a contributing guide, but explains how to write one, something like:

Here is where you should write a guide for new contributors.
Replace this explanation text with your own!

Start welcoming them and explaining that you're open to bug reports and pull requests (if you are).
You could use something like:

> :wave: Thank you for your interest in {{package.name}}!
> Please [open an issue]( {{ URL }} ) if you spot anything wrong.

It's also worth discussing with any collaborators how you want to work together (pull requests and making sure all tests pass).
If you make any decisions about working together you should also write them in here.

> Changes to the code by [pull request]( {{url}}/pulls ) should be reviewed by one other team member and the tests should pass.