Add CONTRIBUTING.md - instruction for contributors

[Kotlin tracks maintenance dashboard ](https://tracks.exercism.io/kotlin) references *CONTRIBUTING.md* by label *CONTRIBUTING guidelines* in the [section](https://tracks.exercism.io/kotlin/master/unimplemented) dedicated to yet unimplemented slugs as the core intruction for appending a task to the track.

This reference is [generated](https://github.com/exercism/tracks-maintenance-dashboard/blob/master/src/components/views/TrackMissing.tsx) 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.

Start by reviewing the generated reference in src/components/views/TrackMissing.tsx and the general track source structure convention mentioned in the issue. Add the missing root-level CONTRIBUTING.md with instructions for appending a task to the Kotlin track, then verify that the maintenance dashboard’s CONTRIBUTING guidelines link resolves correctly.

Written by the indexing model from the issue text.