docs: Add prototype API specification

docs: Add prototype API specification

Add specification for a REST API. It documents a number of endpoints
for the following objects/models:

* Projects
* People
* Patches
* Checks

The specification is provided in OpenAPI format [1]. This specification
can be used to automatically generate REST client libraries and to
validate the implemented API.

The API is currently read-only. It is expected that this will expanded
upon as the API grows.

[1] https://openapis.org/specification

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Signed-off-by: Andy Doan <andy.doan@linaro.org>