Documentation of the API could be more helpful
The README says "Termbox's interface only consists of 12 functions", which is not really accurate (there's 21 functions), and lists just the function names. I think it would be helpful to give argument names (e.g. tb_change_cell(x, y, ch, fg, bg) instead of just tb_change_cell()) and either list all 21 functions, or explicitly say that the listed functions are only the most commonly used ones, but the API has a some other functions too.
In addition, it'd be helpful to mention the expected encoding of chars in tb_change_cell. I think it's wchar_t (because the value is later passed to wcwidth()) but maybe I'm missing something because the actual type is uint32_t instead of wchar_t, perhaps for a reason.