added lesson on using HDF5 in R

This is a modified lesson on HDF5 that I used in the NEON/DC workshop at ESA2014.  I also submitted it to Software Carpentry, and it's gone through a substantial editing process there.  I also modified the data README to provide documentation of the data set that I added.
First mention of the word 'node', should it be introduced earlier when describing the structure of the hdf5 format?