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?

[fzhinkin]

@dkhalanskyjb, WDYT about #587 (comment)?

[dkhalanskyjb]

Honestly, nothing comes to mind. kotlinx.coroutines has some opaque rules (naming conventions for tests, using develop as the target most of the time but not always, etc.), but kotlinx-datetime is very lenient. Tests are named reasonably but without a strong convention, the default branch is always the PR target, the style is also mostly just https://kotlinlang.org/docs/coding-conventions.html.

We could try listing even these aspects, but I'd prefer to avoid adding text with obvious information, to decrease the risk that the contributors get intimidated and skip reading the guidelines entirely. If we witness a contributor making a mistake, we can always update the guidelines later in reaction to that.