Add dependency to README.md

Since commit 54d6472ec67ea73e23b30b1534320fbe637, we need the Maven "shade"
plugin. This is yet another dependency we need to list in README.md...

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>