assert_fail.c: add to readme index

README.adoc

@@ -11495,6 +11495,10 @@ One "downside" of glibc is that it exercises much more kernel functionality on i
 
 Programs under link:userland/c/[] are examples of link:https://en.wikipedia.org/wiki/ANSI_C[ANSI C] programming.
 
+* Standard library
+** assert.h
+*** link:userland/c/assert_fail.c[]
+
 === GCC C extensions
 
 ==== C empty struct