Minimal README.md on each plugin

[guigomcha]

I find it very difficult to use the tool if the plugins do not have a minimal README.md that tells you what they do. I understand that the core definition should be in the user-book but this repo should also have a bit of info on what each repo can do.

[titouanj]

We have tasks about that. It's something we are aware of and that we should work on as soon as possible