add note on clean datasets

notes.txt

@@ -30,3 +30,10 @@ files will be retrieved prior to the command execution. Any content that is modi
 produced by the command should be specified with an -o/--output flag. Upon a run or rerun
 of the command, the contents of these files will get unlocked so that they can be modified.
 
+Important! If the dataset is not "clean" (a datalad status output is empty),
+datalad run will not work - you will have to save modifications present in your
+dataset.
+A suboptimal alternative is the --explicit flag,
+used to record only those changes done
+to the files listed with --output flags.
+