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 <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Link: https://lore.kernel.org/r/178a99[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/ABI/README

···
 	layout of the files below for details on how to do this.)
 
   obsolete/
-  	This directory documents interfaces that are still remaining in
+	This directory documents interfaces that are still remaining in
 	the kernel, but are marked to be removed at some later point in
 	time.  The description of the interface will document the reason
 	why it is obsolete and when it can be expected to be removed.
···
 		sure they are working in a proper way and do not need to
 		be changed further.
 
+
+Note:
+   The fields should be use a simple notation, compatible with ReST markup.
+   Also, the file **should not** have a top-level index, like::
+
+	===
+	foo
+	===
 
 How things move between levels: