DokuWiki core developers are developers with write access to the DokuWiki repository. You don't need to be a core developer to contribute to DokuWiki though. Patches and Pull Requests by other developers are merged by the core devs.
You should have joined the main Mailing List and have successfully committed non-trivial pull requests before.
Existing core devs should keep an eye on new contributors and talk to Andi (directly, IRC or Mailing List) when they think someone should get commit permissions.
Your main responsibilities as a core developer are:
Core developers are expected to use their powers responsibly and carefully. You can merge small PRs (like typos, one-line fixes, translations) right away.
Other PRs should be reviewed by at least two core developers (mark PRs you think are ready to merge with a :+1: comment). You should never merge your own PRs before someone else reviewed them (unless they are trivial as described above).
PRs should adhere to our Coding Style and should contain Unit Tests, if reasonable, before they are merged. Always make sure Unit Tests successfully ran through before merging (Travis will check that automatically).
You are free to create new branches within the main repository. No need to fork into your own account.