Tuleap is a Free and Open Source software developed since 2001 under GPLv2 license. Contributions are welcomed, on Tuleap Core, on plugins, in documentation, etc.
This guide will gives you the insights to start your contributions to Tuleap:
Setting up your environment
Push your code for review to integrators
Understand Tuleap internals
You can also find help for your dev related questions on the chat in the #developers channel.
Documentation of a living tool is hard to achieve and, in case of doubts, the source code is always the reference.
When working with sources, you must look closely to Architecture Decision Records that will give insights on the evolutions of the code base and what is expected / current norm.
Getting started, what you need to know to setup your environment and push your code
Development 101, what you need to know