Completed the 6 tasks in READEME.md

[heidi123]

The passed result:

 $ npm run build && npm run test

> [email protected] build
> tsc


> [email protected] test
> jest --verbose

 PASS  src/stack.test.ts
  LoadBalancerStack
    ✓ should create an autoscaling group (407 ms)
    ✓ should use the correct instance type (144 ms)
    ✓ have the required outbound ports open (143 ms)
    ✓ should autoscale on CPU load (101 ms)
    ✓ should have an IAM role to read from s3 (100 ms)
    ✓ should have a userdata script (83 ms)

Test Suites: 1 passed, 1 total
Tests:       6 passed, 6 total
Snapshots:   0 total
Time:        3.971 s, estimated 6 s
Ran all test suites.