Document package dependency on keytool

[nabadger]

I noticed that we need to install keytool when generating the certs.

This should probably be added as a note under the Development section on the README the operator will error-out without it.

I'm also wondering if there's another more lightweight tool that could be used in it's place since keytool is bundled with JDK. Is there a smaller Go option available (then we could just go get... ? :)