Add a how-to doc on writing DT schema documentation. This gives a
description of each section and details on how to validate the DT schema
file. The DT schema are written using json-schema vocabulary in a YAML
encoded document. Using jsonschema gives us access to existing tooling.
A YAML encoding gives us something easy to edit. The example is
annotated to help explain what each section does.

This example is just the tip of the iceberg, but is it the part most
developers writing bindings will interact with. Backing all this up
are meta-schema (to validate the binding schemas), some DT core schema,
YAML encoded DT output with dtc, and a small number of python scripts to
run validation.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>