Improve README

See here for an example of a good, comprehensive, and well-designed README: https://github.com/clauderic/react-infinite-calendar

### GitHub Short Description
- [x] A short informative sentence for link previews
- [x] Use an emoji in the description to get people's attention
- [x] Add a link to a demo page

### README.md
- [ ] Have a nice logo
- [x] Have an awesome image/animated gif
- [x] Title of your project in plain Capitalized English
- [x] Short description and link to demo page

##### Badges from [Shields.io](http://shields.io/)
- [x] NPM version
- [x] Build status (Travis/Circle CI)
- [x] Dependencies
- [x] Code quality (Bithound or similar)
- [ ] Code coverage
- [x] License

##### Features
- [x] Use an emoji
- [x] Have at least 4 or 5 features
- [x] Keep the focus on simplicity, lightness (e.g. zero-dependency), and ease-of-use

##### Other Sections
- [x] Getting Started or Installation + Usage
- [x] API
- [x] Contributing guidelines