Update README with latest functionality, usage examples and feature roadmap

[AlexWootton]

The current README isn't descriptive of the current or the intended behaviour of the software.

Let's update it to include:

• Short synopsis of functionality (readers should understand the most common/intended use-case for the software)
• Installation requirements/instructions
• Usage examples covering top level functionality (things shown when you run paranoia --help, currently export and inspect)
  • including example output will help potential users evaluate the software without needing to download it
• Feature roadmap (a simple TODO style section at the bottom linking to related GitHub issues would do)
  • this will hopefully give a good indication of the intended direction of development and reduce the number of duplicated enhancement requests