DOC: example of pandas to R transfer of DataFrame using HDF5 file
closes #9636 
I had some trouble getting the HDF5 import of DataFrame objects to work on the R side (pandas is fine). I've added an example to the documentation with a simple load function in R which assembles an R data.frame from the corresponding *_items and *_values nodes in the HDF5 file.

NOTE: Any improvements to the R code are welcome, I'm just starting to work with R. :-)
