Add contribution/branching guide

[runegan]

Original report by Remco Janssen (Bitbucket: [Remco Janssen](https://bitbucket.org/Remco Janssen), ).

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For future contributors…

[runegan]

Original comment by Rune Gangsø (Bitbucket: runegan, GitHub: runegan).

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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

[runegan]

Original comment by Remco Janssen (Bitbucket: [Remco Janssen](https://bitbucket.org/Remco Janssen), ).

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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