Incorperate Readme into Docs

The  readme is great and  contains lost of useful information about how to use the package.
I think we should just run `cp(joinpath("dirname(@__DIR__)/README.md"), joinpath(@__DIR__, "src", "index.md"))`
in the `docs/make.jl`
and move the current `index.md` to be called "API Reference" or similar.