Update and restructure README

There has been some new information in the README and the structuring needs some overhauling.

# Idea: 

```
H1: Name of the project
  H2: Python version
  H2: Installation
    H3 From PyPI
    H3 From source (for development)
  H2: Usage
    H3: Basic Usage
    H4: Extended Usage
  H2: Examples
  H2: Testing
  H2: Contributing
  H2: Authors
  H2: License
```