Document incongruous file contents

Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

tests/case_utils/case_validate/cli/README.md

@@ -0,0 +1 @@
+*Note*: Some files in this directory have contents incongruous with their file extension.  This is a test of whether the `--format` flag overrides the file name detection, so incongruities are expected.