README rewrite

Hey guys,

I just spent some time rewriting our README file. Since it's kind of the project homepage at this point, I figured it'd be worth a little investment. You can [preview it here](https://github.com/angular-ui/ui-router/tree/readme#angularui-router-).

**The highlights, in brief:**
- More cohesion and better flow between sections
- Consistent voice, use of tenses, and overall consistency of style
- Code examples have been updated to reflect best practices and new features
- Top-level navigation lists related pages
- **A section that instructs people on the proper way to report issues**

**Things we still need:**
- All the plunkr examples need to be updated! @timkindberg you've got some kick-ass templates already, but they need to be updated and cleaned up a bit — ping me on IRC if you'd like to work together on this
- We need a real **Contributors** section to better convey project goals & philosophy, expectations and standards for code contributions, roadmap link, etc.

Did I miss anything?