kafka instructions - tell user to enable sidecar injection

[jmazzitelli]

All works, except the kafka quick start instructions here should tell the user to turn on injection (it was mentioned above but not here - if someone just wants the kafka demo, they would need to know to do this).

So I suggest adding this at the start of the kafka instructions (the same instructions as you had above):

If not already done, enable istio injection:

`kubectl label namespace default istio-injection=enabled`

*Note: though not mandatory, it's the main purpose of this demo to use Istio networking rules. The demo will still run without Istio.*