Add CONTRIBUTING.md - instruction for contributors

[eparovyshnaya]

Kotlin tracks maintenance dashboard references CONTRIBUTING.md by label CONTRIBUTING guidelines in the section dedicated to yet unimplemented slugs as the core intruction for appending a task to the track.

This reference is generated and follows the general track source structure convention.

Currently the link does not work, as there is no such an instruction in the root of this repository.

Kotlin track should follow the convention to be fully leveraged my the maintanance dashbord.

[dector]

We should not forget to add test cases naming like here.

[eparovyshnaya]

We should not forget to mention Official Kotlin code style as well