ref-manual/variables: document the CHECKLAYER_REQUIRED_TESTS variable

This was introduced in oecore by commit
ce2e42ace2d15fb6745437cf0a7f07d28398ca12 ("insane: Split ERROR_QA into
CHECKLAYER_REQUIRED_TESTS").

(From yocto-docs rev: 5b205e113015430f6d04e455af7c0d5643863fbc)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -1250,6 +1250,17 @@ system and gives an overview of their function and contents.
       -  :term:`BUILDSDK_CFLAGS` when building for
          an SDK (i.e. ``nativesdk-``)
 
+   :term:`CHECKLAYER_REQUIRED_TESTS`
+      The :term:`CHECKLAYER_REQUIRED_TESTS` variable lists the QA tests that are
+      required to be enabled to pass the Yocto Project Compatible status for a
+      layer. It is meant to be a read-only variable and any change to the
+      variable may be done with the approval of the :oe_wiki:`Technical Steering
+      Committee (TSC) </TSC>`.
+
+      For more information on the Yocto Project Compatible status, see
+      the :ref:`dev-manual/layers:Making Sure Your Layer is Compatible With
+      Yocto Project` section of the Yocto Project Development Manual.
+
    :term:`CLASSOVERRIDE`
       An internal variable specifying the special class override that
       should currently apply (e.g. "class-target", "class-native", and so