Make usage instructions more visible if not enabled

[rogchap]

From @Globegitter

Aaahh, I did not see the

enableDevTools([ client, ]); section. This is not visible in the chrome store or in the plugin itself. Do you think it is possible to add this somewhere more visible so one does not have to read the README here?

We should detect if a page has not been enabled and add instructions to the plugin screen.