@vydas The feature description is not very clear.
I guess that you mean some kind of tag in a note that will link to a text file on disk.
What you can do already is:-
Open a text file in Vivaldi
Select the text that you wish to use as a summary for the note
Type a short title for the note
Now, double-click the note to load the text file from your local drive or from your own server if it is stored there.
0_1537861537465_Note from Text File.png