[Docs] Add documentation for manually testing Fleet Server API endpoints

[kpollich]

Fleet Server's API endpoints (e.g. check in) require an active agent API key and some knowledge of how to provide that API key. We should document the process by which API endpoints can be manually tested either through cURL or an API client (e.g. Postman, Thunder Client, Insomnia, etc) so developers can sanity check updates to API endpoints outside of just unit/integration tests.

These docs should be added to the README.

[jlind23]

@kilfoyle do you believe this is something you can take on?

[kilfoyle]

@jlind23 I think these are internal docs (for the readme). I'm happy to help and/or review, but I'd need someone from development to provide the draft content.