create cheetsheet.md

[uvtc]

First draft at a cheatsheet for djot.

[uvtc]

Argh. Hang on. Github's markdown doesn't support subscript and superscript, and won't pass through the highlighting.

[uvtc]

Github's md won't render the mark tag, and doesn't appear to allow a span with inline css either.

[uvtc]

@jgm , if you don't like the format of this cheatsheet, or the humor in the draft here, or if you'd rather write your own (possibly at a later date), no offense taken if you accept this PR then make massive changes, or if you'd rather not accept this PR at all. I'd just like to see a cheatsheet for djot.

[jgm]

I like the basic idea. ~~I think it would be good to add some tooling, so that the examples are all converted by djot to HTML and put in columns, as in doc/syntax.html. (Then we'd also get correct output for highlighting, among other things.)~~ (Retracting this suggestion.)

[uvtc]

Added a note that you can use tildes instead of backticks. Examples still use backticks (though I think it would look better to use tildes in the examples).