Add a contributing.md

[johanhelsing]

@NavidK0 Just wondering how you prefer things to be done?

Things like:

• Is breaking changes ok (bumping major version of course)? Or do you prefer to avoid this as much as possible?
• Now that I get write access, do you still prefer changes to go through PRs?
• Other things?

Might make sense to put this in a contributing.md.

[NavidK0]

For now:

• Is breaking changes ok (bumping major version of course)? Or do you prefer to avoid this as much as possible?
  We should follow semantic versioning. So if we make a major breaking change (that isn't just removing deprecated APIs), we should increment the major version.
  Introducing breaking changes is fine as long as they are necessary for the health of the project/increase the usability of the project.

• Now that I get write access, do you still prefer changes to go through PRs?
  Not for anything you don't need us to look over or are certain about. If it's a quality of life change that isn't a huge feature or major change, no need to make a PR when you have write access. (Bug fixes shouldn't need PRs either.)

• Other things?
  We should have a development branch (or features branches if preferred), and avoid pushing to the main branch until the feature(s) in development is/are stable.
  Other than that, nothing comes to mind.

I'll be sure to write up a proper CONTRIBUTING.md when I get a chance. Thanks for the help!