Readme.md need to be modified for creating HttpRequests to connect US server #5

I tested facepp-v2 branch and I think the v3 branch has the same issue.

In order to connect to us server, request should be:
HttpRequests httpRequests = new HttpRequests(you_api_key_string, you_api_secret_string, false, false);

The Readme.md should specify the difference between US server and China server in the right way.