Update README.md

modules/hdf/README.md

-HDF I/O
-============================================================
+Hierarchical Data Format (hdf) I/O
+==================================
 
-The module contains I/O routines for Hierarchical Data Formats.
+The module contains I/O routines for Hierarchical Data Format: https://en.m.wikipedia.org/wiki/Hierarchical_Data_Format meant to store large amounts of data. This module does not implement the specs from scratch, it's a wrapper on top of libhdf5, which should be pre-installed by the user.