Make sure you have read the forewords of the section you are going to complete.
Make sure you do not disclose any sensitive information (table names, credentials). Remember that this playbook is public. That's why architecture documentation goes into repositories' wiki.
Fill in every section you have created, do not leave placeholders. There is a backlog hereunder for ideas.
Make your draft reviewed by the CTO before merging it.
Notify the team into the developers channel when you merge a change, so that everybody can immediately enjoy the newly added value.
Here is a list of potential additions, pick an item if you feel to:
[Coding Practices] Peer Programming
[Tutorials] How to write bugs reports
Add a "Snacks Bar" section with funny yet interesting resources about web or development