add structure.h5 from updater test data (#22)

* add structure.h5 from updater test data

* Update README.md

Co-authored-by: Axel Huebl <[email protected]>

---------

Co-authored-by: Axel Huebl <[email protected]>

Author: s-sajid-ali

README.md

@@ -5,6 +5,8 @@ This repository contains examples of [openPMD](http://www.openpmd.org/#/start) d
 - 3D Cartesian
 - Cylindrical with azimuthal decomposition ("thetaMode")
 
+The `structure.h5` is an openPMD 1.1.0 compliant file contains the bare bones structure with no associated data.
+
 In order to obtain these datasets:
 - Clone this repository
 ```