add note datalad and git diff

notes.txt

@@ -11,3 +11,8 @@ If you clone a dataset into an existing
 dataset (as a subdataset), remember to specify the
 root of the superdataset with the '-d' option.
 
+There are two useful functions to display changes between two
+states of a dataset: "datalad diff -f/--from COMMIT -t/--to COMMIT"
+and "git diff COMMIT COMMIT", where COMMIT is a shasum of a commit
+in the history.
+