Expand EXPOSURE_NOTIFICATION_API_USAGE.md

[pdehaye]

I was just told in #71 of EXPOSURE_NOTIFICATION_API_USAGE.md.

There are a few bits of information that it would be good to have in that file:

• Why do you upload 14 days worth of keys, if you only check 10?
• Where is the ExposureConfiguration obtained from, exactly? What is the exact URL used? Does it set parameters like the build number? How often is it fetched?
• When/How often are diagnosis keys fetched from the server?
• Why is there this limitation of testing 20 keys (which seems a severe constraint)? is it because the framework is not exactly used the way it was intended?