Docs: Plugin feature: modifiers and type

[happycollision]

Would you accept a PR that begins to explain how to use type and modifiers in your plugin? I wanted to create

emboss-{size}/{opacity}

and needed to dig into the test files to see an example of how to do it. It might be nice to at least explain the concept in broad terms.

If these things are considered private API, then perhaps they should be removed from the public types? (Seeing { modifier } in the type spec in my IDE tipped me off that something was there.)