README fix: Instructions to prevent plugin banner

[mrmaxsteel]

Fixes #608

[carlossg]

See #310 this will prevent also admin user password generation

[notnarb]

@carlossg Could some sort of README fix be implemented -- if only temporary?

I understand that the proposed solution in this PR may not be desired, but following the README I ended up wasting a considerable amount of time trying to figure out why the setup wizard wasn't being skipped when I did as instructed:

For 2.x-derived images, you may also want to
RUN echo 2.0 > /usr/share/jenkins/ref/jenkins.install.UpgradeWizard.state
to indicate that this Jenkins installation is fully configured. Otherwise a banner will appear prompting the user to install additional plugins, which may be inappropriate.

---

Thanks to your comment I was able to discover from #310 that '-Djenkins.install.runSetupWizard=false' is more-likely what I wanted, but the README is still confusing and misleading. Is there an acceptable change that you see being made for this PR?

[carlossg]

Yes, readme should be changed and that should be the outcome of #310