MAINTAINERS: Add an entry for the code of conduct

As I introduced these files, I'm willing to be the maintainer of them as
well.

Acked-by: Chris Mason <clm@fb.com>
Acked-by: Olof Johansson <olof@lixom.net>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Theodore Ts'o <tytso@mit.edu>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>