Add contribution/branching guide

[Remco Janssen]

For future contributors…

[Rune Gangsø]

Good idea!

What should be included in the guide? What is normally there?

I guess there should be a note about develop being the default branch, and master is only used for release/stable versions.

Also:

• When to use Pull Requests
• When to create issues

[Remco Janssen]

Yes exactly! It doesn't have to be long.

It should also state we use the git flow model. We can explain the main points of it and link to more info. I like this one as it's visual and shows examples: https://www.atlassian.com/git/tutorials/comparing-workflows#gitflow-workflow

Other points are:

• Coding style & ESLint
• Choosing the right module
• Build process

Examples:
https://github.com/jessesquires/HowToContribute https://github.com/codemirror/CodeMirror/blob/master/CONTRIBUTING.md