Add contribution guidelines

[dkhalanskyjb]

No description provided.

[fzhinkin]

For other libraries, like kotlinx-coroutines, we provide a more-or-less detailed overview of what needs to be done before submitting a pull request (like, running tests locally, choosing a target branch for a pull requests, aligning code style with our conventions, etc.).
While we're explicit about not accepting PRs with new features, such information could be helpful for those who's willing to fix bugs or update documentation. Should we include it into the contribution guideline?