add readme for nrrd model source/license

examples/models/nrrd/README.txt

@@ -0,0 +1,14 @@
+# Source and license of the files in this directory
+
+
+## stent.nrrd
+
+A 3D volume converted from stent.npz from the imageio project.
+https://imageio.readthedocs.io/en/latest/standardimages.html
+
+It is in the public domain.
+
+
+## l.nrrd
+
+Unknown