doc/rgw: release note for GetObjectAttributes

Signed-off-by: Casey Bodley <[email protected]>

Author: cbodley

PendingReleaseNotes

@@ -20,6 +20,7 @@
   with their Not and IfExists variants.
 * RGW: Adding missing quotes to the ETag values returned by S3 CopyPart,
   PostObject and CompleteMultipartUpload responses.
+* RGW: Added support for S3 GetObjectAttributes.
 
 * RBD: All Python APIs that produce timestamps now return "aware" `datetime`
   objects instead of "naive" ones (i.e. those including time zone information