livepatch: Add some basic livepatch documentation

livepatch framework deserves some documentation, definitely.
This is an attempt to provide some basic info. I hope that
it will be useful for both LivePatch producers and also
potential developers of the framework itself.

[jkosina@suse.cz:
 - incorporated feedback (grammar fixes) from
   Chris J Arges <chris.j.arges@canonical.com>
 - s/LivePatch/livepatch in changelog as pointed out by
   Josh Poimboeuf <jpoimboe@redhat.com>
 - incorporated part of feedback (grammar fixes / reformulations) from
   Balbir Singh <bsingharora@gmail.com>
]
Acked-by: Jessica Yu <jeyu@redhat.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

MAINTAINERS

@@ -6605,6 +6605,7 @@ F:	kernel/livepatch/
 F:	include/linux/livepatch.h
 F:	arch/x86/include/asm/livepatch.h
 F:	arch/x86/kernel/livepatch.c
+F:	Documentation/livepatch/
 F:	Documentation/ABI/testing/sysfs-kernel-livepatch
 F:	samples/livepatch/
 L:	live-patching@vger.kernel.org