User-contributed notes per method/class/module

[marcoroth]

Similar to what APIDock has:

[faqndo97]

@marcoroth I would like to tackle this one. In order to get the same behavior we need to have Users, plus all the authentication. Then for comment, how "advance" we want this feature? just comment? should a comment be able to reply to another comment? tag users? Also, for the body would you like allow some markdown or just action text or similar disabling a few formatting options like titles for example? Let me know, thanks!