Add contribution guidelines

[pinn3]

While making the changes for #129 I felt that there may be a need for this.

By the looks of the commit log, the format of the messages varies quite a lot. I'm also wondering about what the preferred merge strategy is, like would it be appreciated if one was to keep their branch up to date with master through rebasing? I'm not sure if this is something that is valued here, it might just be me that is overly picky about such things.

Other than that, clarifying the goal with this mocking library might be beneficial. I mentioned in #129 that the changes only seem to relevant for Chrome extensions. Should such cases be supported by this project, or should anything outside of the actual WebExtensions API be omitted?

[RickyMarou]

FYI the repo has moved over to https://github.com/RickyMarou/jest-webextension-mock/

That's a great suggestion that is on my radar, i already made the move to use changesets so it'll be easier to write changelogs for this project.

Essentially what I want to do is:

• Clarify the goal of the library (i.e. it's mocking webext apis, not strictly sticking to the specs but more to the real world of what is available in browsers, especially chrome)
• Write contribution guidelines
• Better automation in general
  • Redo the npm publish automation (I disabled the current one it was using a github action that has not been used for many years and I want to increase security a bit)
  • Automatically git tag and create github releases
  • Maybe enforce conventional commis on some level, albeit i'm not particularly fan of those