One off Notes can be added to your Customers Quotes using the Notes section, these notes will appear on the Quote PDF. Notes can be added when creating the note and also add to an exiting quote. 

In the Web Portal, go to CRM > Quotes > select the Edit pen for the quote > enter details under Notes > Save at the bottom of the quote.

To add a note in the App, go to Accounts > Quotes > select Add Note just below History > enter details/comments > Add Note to Save.

You can add Notes to a new quote by selecting Add Note when creating it as well.