Enhance 7t_trt example with demonstration of "entities-less" physio.json for inheritance principle

We already have a single dataset with similar entities less
file: ds000248/T1w.json . I decided to enhance this one as it is
a nice demonstration of allowance here since there is another metadata
file but with a different suffix

Author: yarikoptic

7t_trt/README

@@ -1 +1,5 @@
 7t_trt data
+
+Demonstrates:
+
+- inheritance principle Example 5 on use of a single entities-less `physio.json`