Add maintainers guide

[adrinjalali]

We should separate maintainers' and contributors' guide, and we can add this list as prefixes to commit titles:

• DOC: documentation changes
• FEAT/FEA: new major features
• ENH: enhancements to existing features
• CI: continuous integration, sometimes overlaps with MNT
• MNT/MAINT: maintenance, technical debt, etc
• FIX: bug fixes
• TST: new tests, refactoring tests
• PERF: performance improvements

[BenjaminBossan]

(addendum from comment somewhere else: ENH should have user-facing implications)