add existing documentation

doc/sample/Sum.im

@@ -0,0 +1,29 @@
+package net.doorstuck.test
+
+cout put:'Finds the sum of a set of numbers.'
+
+sum = 0
+
+while true do
+    cout print:'Number (blank to finish): '
+    cout flush
+
+    input = ''
+    v = 0
+    cin get:input
+
+    if input == '' then
+        break
+    end
+
+    try
+        v = Int parse:input
+    catch (#err:number_format, err)
+        cout put:'{input} is not a valid number.'
+        continue
+    end
+
+    sum += v
+end
+
+cout put:'Sum: {sum}'