Planning a CONTRIBUTING.md

The crate is quite famous and contributions abundant, so it would be nice to have a guideline on contributions.

# Request

I propose to add the following.
- File `CONTRIBUTING.md` in the root that can guide new contributions.
- Template for GitHub issues

# Details

## CONTRIBUTING.md

I propose the following structure of the file
- Prelude
- Rust code
- User-guide [nalgebra.org](https://www.nalgebra.org/)

### Prelude

It should contain
- Thanks for contributing to the project.
- A list of ways to contribute with relevant links

### Rust code

It should contain a step-by-step guide from forking to submitting a PR. I suggest adding a link to the [official Github PR guide](https://docs.github.com/en/github/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests) too.
Some things which are nice to have in the step-by-step:
- Details on how to run tests, clippy, rustfmt
- Contact information for urgent PRs
- Mention that the PR process may iterate further

### User-guide [nalgebra.org](https://www.nalgebra.org/)

It should contain
- Link to the source of the website
- How to simply notify, compared to PRs
- Step-by-step, in case one wants to do the PR
- Contact information for urgent PRs
- Mention that the PR process may iterate further

## Template for GitHub issues

I propose at least the following categories.
- Compile issue
- Feature request
- Bug report
- Question
**Note:** More experienced contributors might have a better idea of these categories.

Also, it would be nice to have automatic Github labels (I do not know how to do this).

# Inspiration

As @sebcrozet mentioned, [rapier CONTRIBUTING.md](https://github.com/dimforge/rapier/blob/master/CONTRIBUTING.md) is a good example.

For templates in Github issues, [rand](https://github.com/rust-random/rand) is a nice example :)