GSoD: A new open-source program

Google recently announced new open-source program, [Google Season of Docs (GSoD)](https://opensource.googleblog.com/2019/03/introducing-season-of-docs.html) which is similar to current GSoC but its just all about documentation. Even though coala at its current state is well documented, there are still various things which are not very well documented.

I believe coala should participate in GSoD since it has been participating in GCI and GSoC from quite a few years. The stipend is similar to GSoC if not the same. This provides a lot of opportunities for projects to have a very good base for documentation.

I'm not sure if coala would participate this year (2019) since the maintainers are not very active and there are not many ideas introduced for the students to propose a project on. But, in the coming years I think we need to get this set up.

cc. @jayvdb @Makman2