CONTRIBUTING.md: Update about 'struct tok' usage

[skip ci]

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 77f180e40584b6bbcadfbc6eb391664493fba761..67ce034b6d32747b75c87e3ea54f755c909d72a3 100644 (file)
--- 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.