Add note on adding siblings

notes.txt

@@ -64,3 +64,13 @@ 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.
 
+To update from a dataset with a shared history, you
+need to add this dataset as a sibling to your dataset.
+"Adding a sibling" means providing DataLad with info about
+the location of a dataset, and a name for it. Afterwards,
+a "datalad update --merge -s name" will integrate the changes
+made to the sibling into the dataset.
+A safe step in between is to do a "datalad update -s name"
+and checkout the changes with "git/datalad diff"
+to remotes/origin/master
+