CONTRIBUTING.md: Update about 'struct tok' usage

fxlb · fxlb · commit f6b74c5cf232 · 2023-05-23T16:46:23.000+02:00
[skip ci]
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
@@ -202,6 +202,7 @@ and ask!
 
 *  Use `struct tok` for indexed strings and print them with
    `tok2str()` or `bittok2str()` (for flags).
+   All `struct tok` must end with `{ 0, NULL }`.
 
 *  Avoid empty lines in output of printers.
 
