docs: add step-by-step guide that describes how to add new checker

[quasilyte]

In short, the process is like:

1. Come up with checker idea. Concentrate on what it checks.
2. Select one of the base checkers kind (example: expr checker, stmt checker, etc.).
3. Define checker type and constructor function.
4. Add entry to checkers list in lint.go. It could be a good idea to mark recently added checker as experimental.
5. Add test directory that is named after the checker in cmd/kfulint/testdata.
6. Add checker_tests.go file in that directory and add some positive and negative tests there.
7. Run tests. They must fail as your checker does not check anything yet. Tests can be run with
   go test -v github.com/PieselBois/kfulint/cmd/kfulint.
8. Implement checker itself. Make tests pass.

[weeellz]

@quasilyte, where should this guide be? In contributing.org or in separate file?

[quasilyte]

Tough question.
I think either way is fine.
As soon as we get good content it will be trivial to move it around.

By the way, are you OK with org files?
Does your editor support them?

[quasilyte]

I suggest assigning @tengufromsky for this doc review.
He can check if it can be understood by someone who is not very familiar with our project.

[weeellz]

@quasilyte, yup, org files is ok.