Add more precisions about what is a "Mature project"

**I would like to ...**

- [x] to add content

**What is the section concerned ?**
Preparation step

**What is the point concerned ?**
Make sure your project is mature enough

**What would you like to add or modify ?**
I think we should be a bit more precise about what is a "mature enough" project. Maybe we could add some more precision about it ?