Added additional example and info.

example3.c

@@ -0,0 +1,35 @@
+#include "tinyexpr.h"
+#include <stdio.h>
+
+
+/* An example of calling a C function. */
+double my_sum(double a, double b) {
+    printf("Called C function with %f and %f.\n", a, b);
+    return a + b;
+}
+
+
+int main(int argc, char *argv[])
+{
+    te_variable vars[] = {
+        {"mysum", my_sum, TE_FUNCTION2}
+    };
+
+    const char *expression = "mysum(5, 6)";
+    printf("Evaluating:\n\t%s\n", expression);
+
+    int err;
+    te_expr *n = te_compile(expression, vars, 1, &err);
+
+    if (n) {
+        const double r = te_eval(n);
+        printf("Result:\n\t%f\n", r);
+        te_free(n);
+    } else {
+        /* Show the user where the error is at. */
+        printf("\t%*s^\nError near here", err-1, "");
+    }
+
+
+    return 0;
+}