Improve README

[slang25]

The README could do with some sprucing up, a lot of content was moved into the docs and now things are looking a bit bare. I'd like the readme to give first time visitors a positive impression of the library, and an intuition as to what it does and doesn't do, within a few seconds of scrolling through.

Ideas for things to add:

• A brochure style section (i.e. a clear and visual elevator pitch)
• A features list
• Instructions for running integration tests (in addition to the contributor.md)

Further inspiration can be found looking here readme.so

A nice header would be cool:

(@gkinsman you had some good examples of other READMEs, could you add links for reference)

[stale]

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