Update README to reflect $schema detection

Author: handrews

README.md

@@ -103,6 +103,8 @@ The precise steps described do not need to be followed exactly, but the results
 
 To test a specific version:
 
+* For 2019-09 and later published drafts, implementations that are able to detect the draft of each schema via `$schema` SHOULD be configured to do so
+* For draft-07 and earlier, draft-next, and implementations unable to detect via `$schema`, implementations MUST be configured to expect the draft matching the test directory name
 * Load any remote references [described below](additional-assumptions) and configure your implementation to retrieve them via their URIs
 * Walk the filesystem tree for that version's subdirectory and for each `.json` file found: