update docker documentation

Your docker image exists here:

https://hub.docker.com/r/neurodata/ndio/

You should create a readme and documentation file for it that match the specification established here:

https://hub.docker.com/r/neurodata/template/

P.s. @wrgr I don't know if you should do this or someone else, but I figured you'd make that decision so assigned you for now.
