Add readme to doctest

[ysimonson]

Just came across this in glob: https://github.com/rust-lang/glob/blob/7b29204bb108a83300b0ce877072fcb62b83316a/src/lib.rs#L70

Looks like a great way to ensure the examples in README are up to date with the latest, whereas at the moment, on breaking changes I hack something together on the README and hope it works.

For examples in the README that can't run via doctests (because it's in python or because it's using gRPC), we'll need to add an attribute to ignore testing: https://doc.rust-lang.org/rustdoc/write-documentation/documentation-tests.html#attributes