contributing doc: describe cnitool installation
contributing doc: describe cnitool installation

cnitool must be installed before running tests because cnitool is
invoked during the tests

Signed-off-by: Sho SHIMIZU <e7963d3963915f0d9e2a8f094e7a1310d7e81dae@gmail.com>

containernetworking/plugins,containernetworking/plugins