Add documentation for XML format

[gtbX]

Description

Added an xml.md that describes the XML schema as I understand it.

Related Issues

Issue: #25

How Has This Been Tested

n/a

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

Screenshots (if appropriate)

n/a