Add a Quick start section to the README

I can never remember what value I'm supposed to use for the endpoint when I want to use Mimic. Having a curl and OpenStack CLI example shows the proper value for OS_AUTH_URL settings and how to see the service catalog to get a list of other product-specific endpoints.
also include the following here:
```
curl http://localhost:8900
>> To get started with Mimic, POST an authentication request to:
>> /identity/v2.0/tokens
```