DOC: small user manual

src/core/kutils.c

@@ -155,3 +155,17 @@ void k_beep_process(void) {
     }
 }
 
+char *k_strstr(const char *haystack, const char *needle) {
+    if (!*needle) return (char *)haystack;
+    for (; *haystack; haystack++) {
+        const char *h = haystack;
+        const char *n = needle;
+        while (*h && *n && *h == *n) {
+            h++;
+            n++;
+        }
+        if (!*n) return (char *)haystack;
+    }
+    return NULL;
+}
+