In reply to Dinsan.

If you're interested in writing technical documentation, we can always use more help writing inline docs for WordPress core. If you're interested in writing developer-facing documentation, the docs team is still writing and editing the first version of the official Theme Developer Handbook. The best place to start for either effort (or something else) is to join the #docs channel on the official WordPress Slack team. Visit https://make.wordpress.org/chat to get started.