README.md: Add maven as dependency to compile on Debian

Without maven, "/bin/sh: 1: mvn: not found" error is shown when trying to
compile.

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Rodrigo Campos <rodrigo@sdfg.com.ar>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>