Simply put, the node needs a "notes" (or s/n "nts") attribute first, then its plain sailing (manually writing a note creates the attr, which then stores the string - or so im assuming, looks like that's handled through a scriptJob)
So to add a note to a node, first check whether it already has an attribute called "notes", create one if not, then write something.
string $node[]=`polyCube` ;
if(!`attributeQuery -node $node[0] -ex "notes"`)
addAttr -ln "notes" -sn "nts" -dt "string" $node[0] ;
setAttr -type "string" ($node[0]+".notes") "This node now has a Note. You can store various bits of information here." ;
(The note on this script example is on the transform node of the polyCube object ($node[0]) as $node[1] would be the shape object)