Add XML coding standards

[PierceAndy]

• Use 2-space indentation Coding standards: Use 2-space indentation for markup languages #77

Reason: Markup languages tend to have deep nesting.

• Space before self-closing tag (i.e. />)

Reason: Readability.

• Line wrap at so & so number of characters (to be defined).
• Header comments for code block, should be inside or outside?

[PierceAndy]

Possibly add a sub-section specifically for checkstyle.xml files.

• Properties with default values that aren't changed shouldn't be included (e.g. severity property has default value of error. If using the default value, don't include it. Discussion here and here)
• If using the same module multiple times, don't omit properties with default values if these properties are different across modules with the same name (Discussion here)
• One value per property (e.g. from official checkstyle configuration)

[yamgent]

Possibly add a sub-section specifically for checkstyle.xml files.

That problem does appear outside checkstyle.xml actually. (e.g. JavaFx's FXML files).