docs: dt: writing-schema: mention yamllint

The dtschema uses yamllint, if present, to check the syntax of the
files, so document this.
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220419084942.27409-1-krzysztof.kozlowski@linaro.org

Documentation/devicetree/bindings/writing-schema.rst

@@ -134,6 +134,8 @@ installed first. On Debian/Ubuntu systems::
 Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be
 installed. Ensure they are in your PATH (~/.local/bin by default).
 
+Recommended is also to install yamllint (used by dtschema when present).
+
 Running checks
 ~~~~~~~~~~~~~~