docs: ABI: README: specify that files should be ReST compatible

docs: ABI: README: specify that files should be ReST compatible

As we plan to remove the escaping code from the scripts/get_abi.pl,
specify at the ABI README file that the content of the file should
be ReST compatible.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/178a997070debd1953ba7d302c375948501d6193.1604042072.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>