FSAL_LUSTRE/Upcalls : adds a HOWTO file

Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>
FSAL_LUSTRE/Upcalls: the thread can see changelog records

Change-Id: I8ded710b577a949486e16e14d12266f1ee452aa7
Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>