Updated Storage backend compatibility (markdown)

Storage-backend-compatibility.md

@@ -5,6 +5,7 @@ Some storage backends do not support all the features of S3:
 * does not return meaningful ETag
 * does not support multi-part upload
 * only supports object listing when delimiter is "/" -- does not support recursive listing
+* only supports object listing with prefixes when the prefix matches a directory name
 
 ## Azure