Add links to development policies & guidelines to CONTRIBUTING.md

[cr7pt0gr4ph7]

I have read CONTRIBUTING.md when I started developing with Mixxx, but have since learned through various (albeit very friendly :)) comments in issues and reviews that there are a few important documents, like:

• Minimum requirements policy
• Coding guidelines

that are not linked there, but which new contributors-to-be should probably be made aware of as soon as possible.

@JoergAtGithub @daschuer @ronso0 @Holzhaus @Swiftb0y @fwcd @acolombier @uklotzde (basically everyone who was active in the last few months)
Are there any other important documents or pieces of information that should be linked there?

Thanks in advance for any helpful pointers!

[daschuer]

Maybe:
https://github.com/mixxxdj/mixxx/wiki/Internationalization
and
https://github.com/mixxxdj/mixxx/wiki/Release-Checklist-2.4.0

[cr7pt0gr4ph7]

Just as an aside: Would it make sense to have a dedicated GitHub label for this and similar issues related to development & contributing?

For example:

• developer-experience: Issues, bugs and PRs related to the development process, development environment & developer documentation of Mixxx itself.

[daschuer]

Done