Add overall information on the issue/pr management approach

@kamyshova please add the following information on the overall contribution to the repo and how the issues are tracked:
* Entrypoint:
* `Issue`: if there is only an idea for some feature and fix and it shall be discussed first
* `PR`: if there is already an implementation for some feature or a bug-fix
* What information shall be provided to the Issue/PR description (we need to add Issue/PR templates further), which branch shall be used as a merge destination, how to link PRs and issues
* If the issue is created first - how it gets implemented (discussion -> decision)
* If a PR is created first (or a PR is created as a result of work on an issue) - how to request to approval, how it gets discussed, approved, merged
* Any other valuable information of your choice