Updated README and documentation-rs

Improved db seed and changed logic for who is allowed to make comments

- Min/Max dates are now calculated based on `DateTime.Now` instead of fixed / static dates
- Only users with at least one `Completed` reservation state is allowed to make comment (as opposed to `Completed` or `Denied` before (which was required in project specification))

Merge branch 'master' of https://github.com/DaniloNovakovic/apartment…

…-reservation