Domain documentation

- Domain classes diagram (available at https://www.draw.io/?#G0ByyXLa89xHUdTEl0MktFWnk2LUE).
- Itroduction to system domain design.
- Explanations for specific domain sections of the application:
    - Job searches
    - MyMenu
    - Employees
    - Applicants
    - Vacation reports
Merge pull request #336 from AlphaGit/domain-documentation

Domain documentation