Add Fluent for Developers Guide

[zbraniecki]

Similar to Syntax Guide there is a need for Fluent for Developers Guide.

[zbraniecki]

I think we'll need a guide per stack - Rust, React, JS, Web, Python for now.

[stasm]

I created Good Practices for Developers on the wiki based on our experience in bug 1411012. Perhaps it can serve as a base for the guide or complement it.