ref-manual: add section for create-spdx class

create-spdx was added in the honister release. This is a stub since I
unfortunately don't have time to fully document this class and related
variables, hopefully someone else can expand it in future.

(From yocto-docs rev: 5a4aa9dca1468d56ab5ec2c3a5ebf1ea364df5d0)

Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/classes.rst

@@ -368,6 +368,14 @@ authors used.
 Both build methods inherit the ``cpan-base`` class for basic Perl
 support.
 
+.. _ref-classes-create-spdx:
+
+``create-spdx.bbclass``
+=======================
+
+The ``create-spdx`` class provides support for automatically creating
+SPDX SBoM documents based upon image and SDK contents.
+
 .. _ref-classes-cross:
 
 ``cross.bbclass``