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What do you mean by READMEs here? Markdown readmes support embedding images from external files, which isn't really useful if you're looking at the readme directly (or do you use data: uris there? Last I looked they had too-severe length limits.)

(I'm also curious what you mean by SMS, or are you conflating it with RCS?)

Messaging apps don't have fixed width apps, I think. You would need to pick one app's specific font (idk if Monodraw lets you do that), and draw using that.

You can use it for mocking up a text user interface.