Rewrite README

[egpbos]

Take inspiration from the other xtensor pages, e.g. xensor-io has a nice compact one, that would be good as a header. The rest can be something like:

1. Intro
2. Example
3. Installation
4. Dependencies
5. Usage (i.e. more examples, or links to notebooks/binder)
6. Building and running the tests
7. License

[egpbos]

Still todo:

• [x] A bit more introduction (see e.g. https://github.com/QuantStack/xtensor-io#introduction)
• [ ] More usage examples and links to notebooks / binder (#18)