Update README: reflect abstract state view rule.

Submitting this change in hopes it might prevent someone else going mad when their views templates don't load. (Especially those that might copy/paste...)

As per the rule on abstract state views: "Abstract states still need their own <ui-view/> for their children to plug into. So if you are using an abstract state just to prepend a url, set resolves/data, or run an onEnter/Exit function, then you'll additionally need to set template: '<ui-view/>'"