Add some of the information from the docs to the readme.

Hello, I noticed the index.html file in the /docs directory has some good information about using this directive. Would it be possible to get some of this information added to the readme so it is displayed more prominently when viewing the github repo?

With this pull request I attempted to follow the structure if the [ui-date](https://github.com/angular-ui/ui-date)'s README.md file.