Update the readme.md as it was very out of date!

- Remove refs to version 1.0 and version 2.0
- Don't mention so many version numbers - latest usable version will be the one on nuget. Link to the nuget page!
- Talk about semver and the upgrade from v3 to v4 and async

- Take a lot of cues from https://github.com/raoulmillais/node-7digital-api/blob/master/README.md
- start with "about 7digital", "What is this code" and "you will need"
- mention and link to the api key signup microsite, about.7digital.com and developer.7digital.com
- be clear about nuget install instructions and nuget package name