Update Readme.md with IAM permissions information and add buildspec.yaml file supporting deployment test with CodeBuild

[linhlam-mp]

Issue #, if available: #34

Description of changes: Update Readme.md with information about IAM permissions and add buildspec.yaml file supporting deployment test with CodeBuild

Test results:

1. Unit: N/A
2. Integration: N/A
3. End-to-end ("e2e"): PASSED
   1. Verify the stack is deployed successfully.: done
   2. Verify URL endpoint is up and running with no error. done
   3. Subscribe to the product successfully and can visit landing page with no error. done
   4. Create an account using landing page. N/A
   5. Receive notification as a Seller. done
   6. Receive notification as a Buyer. N/A
   7. Verify new user record is created in dynamo DB. N/A
   8. Other: (Depend on the new feature added that the test cases above are not covered)