add note on git annex whereis

notes.txt

@@ -56,3 +56,7 @@ registered subdatasets underneath a subdataset, so a safer
 way to proceed is to set a decent --recursion-limit:
 "datalad get -n -r --recursion-limit 2 <subds>"
 
+The command "git annex whereis PATH" lists the repositories that have
+the file content of an annexed file. When using "datalad get" to retrieve
+file content, those repositories will be queried.
+