document nl_langinfo() check

README.md

@@ -97,7 +97,8 @@ provided, and can be accessed as `notcurses`.
 Before calling into notcurses—and usually as one of the first calls of the
 program—be sure to call `setlocale(3)` with an appropriate UTF-8 `LC_ALL`
 locale. It is usually appropriate to use `setlocale(LC_ALL, "")`, relying on
-the user to properly set the `LANG` environment variable.
+the user to properly set the `LANG` environment variable. notcurses will
+refuse to start if `nl_langinfo(3)` doesn't indicate UTF-8.
 
 notcurses requires an available `terminfo(5)` definition appropriate for the
 terminal. It is usually appropriate to pass `NULL` in the `termtype` field of a