Implement test "add" endpoint to check body parsing and response

Now we know whether we can read and write JSON properly!

index.js

@@ -12,3 +12,8 @@ app.listen(5000, () => {
 app.get("/", (req, res) => {
   res.send("Hi!")
 })
+
+app.post("/add", (req, res) => {
+  const { a, b } = req.body
+  res.send({ result: a + b })
+})