p.haul: add note about patches subject prefix to readme

Add note about p.haul patches subject prefix and example of the
command to set subject prefix for local repository to readme file.
Subject prefix needed since we are going to use patchwork for criu
and p.haul and it is needed to separate patches for each project.

Signed-off-by: Nikita Spiridonov <nspiridonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

README.md

@@ -31,8 +31,11 @@ wiki (http://criu.org/Category:P.Haul).
 How to contribute
 =======
 
-The p.haul patches should be sent to CRIU development mailing list which is
-located at https://openvz.org/mailman/listinfo/criu
+The p.haul patches should be sent to CRIU development mailing list
+(https://openvz.org/mailman/listinfo/criu) with "p.haul" prefix.
+Configure your local git repository using following command to
+set subject prefix automatically:
+* $ git config format.subjectprefix "PATCH p.haul"
 
 Before sending patches please make sure your code formatted according to
 project coding style (we use [PEP8](https://www.python.org/dev/peps/pep-0008/)