adding example file for hdf-to-txt conversion

[alex-koe]

This is an example which converts directly hdf files (no openPMD) to txt file readable by GPT. (see hdf-to-txt folder)

[ax3l]

Let's add a simple README.md in the hdf5-to-txt/ directory when we merge this, to clarify that this is a separate example.

We want to avoid confusion because openPMD HDF5 is also a thing :)