Updated Storage backend compatibility (markdown)

Storage-backend-compatibility.md

@@ -34,6 +34,7 @@ Some storage backends do not support all the features of S3:
 
 * does not allow arbitrary list markers, partially emulated by mapping the last key to the opaque marker
 * does not return ETag as an MD5 hash, instead returning opaque data
+* does not support listing multipart uploads
 * limits multi-part uploads to 32 parts
 
 ## OpenStack Swift