README: Long overdue update

[liskin]

Description

• "community-maintained" is more accurate and sounds better than "third-party"
• refresh description
• add logo and badges to make it prettier (inspired by https://twitter.com/TechnoEmpress/status/1451322423790473218)
• update doc links
• LICENSE tweaked to make it detectable by GH (needed for the badge)
• add CONTRIBUTING to make GH show a link to it when opening issues/PRs

Rendered: https://github.com/liskin/xmonad-contrib/blob/readme/README.md

Checklist

• [X] I've read CONTRIBUTING.md

• n/a I've considered how to best test these changes (property, unit, manually, ...) and concluded: XXX

• n/a I updated the CHANGES.md file

[TheMC47]

Awesome 🎉 the license is still not identifiable though

[liskin]

the license is still not identifiable though

It is identifiable, but the badge won't update until it's merged, and GitHub won't detect it unless it's in the default branch (I did check this by switching the default branch of my repo, and then switched it back to dotfiles).

[TheMC47]

Alright, LGTM