add contributor guidelines to project

[amacado]

I'm missing a guideline for contributions which answers following (and more) questions:

• to which branch a new pull-request should be openend (v3/master/..)?
• commit message requirements
• naming conventions (branches but also variables)
• information on testing process
• information on build process

see https://help.github.com/en/articles/setting-guidelines-for-repository-contributors

[TimonVS]

Good one!

I'm going to attempt to answer a few questions here, but I'll document them properly later.

to which branch a new pull-request should be openend (v3/master/..)?

I'm following these release guidelines: https://github.com/actions/toolkit/blob/master/docs/action-versioning.md. PRs should always be opened against master.

commit message requirements

For now there are no requirements, the project is still very small :)

naming conventions (branches but also variables)

We should add some basic ESLint config for this. There already is a Prettier config but it only works if a developer has the Prettier plugin installed in their editor. We should also add some basic CI using GitHub Actions which runs ESLint agains the code in a PR.

information on testing process

Yup, definitely agree that this should be documented.

information on build process

I'm following the guidelines about ncc provided here: https://help.github.com/en/articles/creating-a-javascript-action#commit-and-push-your-action-to-github. I will document this properly :)