Add solutions for part 1

projects/09/Average/Main.jack

@@ -0,0 +1,27 @@
+// This file is part of www.nand2tetris.org
+// and the book "The Elements of Computing Systems"
+// by Nisan and Schocken, MIT Press.
+// File name: projects/09/Average/Main.jack
+
+// Inputs some numbers and computes their average
+class Main {
+   function void main() {
+      var Array a; 
+      var int length;
+      var int i, sum;
+
+      let length = Keyboard.readInt("How many numbers? ");
+      let a = Array.new(length); // constructs the array
+     
+      let i = 0;
+      while (i < length) {
+         let a[i] = Keyboard.readInt("Enter a number: ");
+         let sum = sum + a[i];
+         let i = i + 1;
+      }
+
+      do Output.printString("The average is ");
+      do Output.printInt(sum / length);
+      return;
+   }
+}