update README

Signed-off-by: Thomas Naughton <[email protected]>

Author: naughtont3

README.md

@@ -12,3 +12,8 @@ to triage the issues as appropriate.
  - [Frontier](https://www.olcf.ornl.gov/frontier)
  - [Crusher](https://docs.olcf.ornl.gov/systems/crusher_quick_start_guide.html)
 
+## Submit an Issue
+
+Please submit [issues](https://github.com/OMPI-X/platforms/issues) for
+supported platforms using the issue tracker.
+