One of the consistent problems with formatting is getting it to work on iOS. Accordance along with many others have chosen to go down the plain text route.
Could I ask you to consider Markdown or Multimarkdown? The markup languages are simple and in a plain text format. Many notes apps on iOS have gone down this route. They allow simple markup of text and then it can be rendered. The note can be edited in plain text and then rendered to reveal the formatting. This would be much simpler than trying to create an RTF format plus it drastically reduces file size. Markdown is also viewable on nearly every computer OS available and the beauty of it is that you can edit your plain text document in any programme you like. It would also solve the problem of editing notes on iOS that were created on the Mac and losing the formatting as they are simple plain text documents. Multimarkdown would be better as it includes things such as footnotes.