Its built in, open the attribute editor and type notes to any node. If you dont want to do it thisway just store the info with fileInfo in your file. You can make the tools needed in less than 5 minutes honestly.
For example to find all the nodes that have notes assosciated with them whould be as simple as:
global proc int filterNodesWithNotes( string $name ){
int $ret = attributeQuery("-exists", "-n", $name, "notes");
return $ret;
}
{
OutlinerWindow;
textField -edit -text "Showing nodes with notes" FilterField;
string $noteFilter = `itemFilter -byScript filterNodesWithNotes jooNotesFilter`;
outlinerEditor -e -f $noteFilter outlinerPanel1;
}
This makes it easier to browse (note the list WILL be empty if theres no notes in the scene, make some in the attribute editor). But yes if you want to make your own editor again no biggie 5 minutes of work and storing in file info is a nobrainer. 5 more minutes and its nice and 5 more minutes and you can itegrate it with your version control system.
No plugins needed just very rudimentary scripting skills.