add note about datalad update

notes.txt

@@ -60,3 +60,7 @@ 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.
 
+To update a shared dataset, run the command "datalad update --merge".
+This command will query its origin for changes, and integrate the
+changes into the dataset.
+