docs(examples): add example programs

examples/fib.lumen

@@ -0,0 +1,12 @@
+// Recursive Fibonacci, printed for the first ten indices.
+
+fn fib(n) {
+    if (n < 2) {
+        return n
+    }
+    return fib(n - 1) + fib(n - 2)
+}
+
+for (i in range(0, 10)) {
+    println("fib(" + str(i) + ") = " + str(fib(i)))
+}