Add step about CODEOWNERS in README (#1161)

Author: ellahathaway

README.md

@@ -26,7 +26,9 @@ There will be a need for modifying existing Dockerfiles or creating new ones.  F
 
 1. Edit Dockerfiles
     - Add/Update the Dockerfile(s)
-    - If new Dockerfile(s) were added, then update the [manifest](#manifest)
+    - If new Dockerfile(s) were added:
+        - Update the [manifest](#manifest)
+        - Update the [CODEOWNERS](./CODEOWNERS) with the respective team code owner(s) (not individual users) for the Dockerfile(s). A team code owner must be assigned to each Dockerfile for maintenance and issue assignment purposes. 
 
 2. Validate the changes locally by running [build.ps1](./build.ps1).  It is strongly suggested to specify the `-DockerfilePath` option to avoid the overhead of building all the images.