scripts/get_abi.pl: add support to parse ABI README file

scripts/get_abi.pl: add support to parse ABI README file

The Documentation/ABI/README file is currently outside the
documentation tree. Yet, it may still provide some useful
information. Add it to the documentation parsing.

As a plus, this avoids a warning when detecting missing
cross-references.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/f1285dedfe4d0eb0f0af34f6a68bee6fde36dd7d.1739182025.git.mchehab+huawei@kernel.org