Document the use of dropins folder

[lesquoyb]

Is your request related to a problem? Please describe.
To my knowledge there's no documentation on how to use the dropins folder, and it's a plugin installation method that is only known by some of the gama developer. Even if its use doesn't go beyond weird plugin debugging, I think that it's a good idea to document it somewhere so the information doesn't get lost.

Describe the solution you'd like
Write a page describing what is the dropins folder purpose and how to use it. Maybe a recipe ?