Add Windows Vagrant file

[koppor]

This adds a new Vagrant VM to get a clean Windows environment with git, a JDK, Firefox and LibreOffice installed. Then one can test JabRef's functionalities more easily.

Also reworked the README.md file(s).

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.