added HDF5 lesson to intermediate R curriculum

Per Greg's request I've added in the lesson for working with HDF5 in R and an associated data set.
You have
    The package we’ll be using in rhdf

I think that should be "is rhdf5".

Looking good otherwise.  I'd suggest using a smaller first file so that your h5ls doesn't return so much output.  You could also show how to create hdf5 files.