Add roi plugin design issue to Readme

Author: dominikl

README.md

@@ -14,4 +14,5 @@ for more details.
 - [OME005: TIFF and OME-TIFF sub-resolution support](OME005/)
 - [OME006: ROI drawing controls](OME006/)
 - [OME007: Viewer controls](OME007/)
+- [OME008: CLI ROI plugin](OME008/)