Document missing OpenAPI/Swagger-Defintion/API-Spec

One thought would be if we can generate this client.
An API Spec file would be required for it.
A few discussions have been ongoing, but no progress/merge yet regarding this topic.
See:

* https://github.com/CachetHQ/Docs/issues/9
* https://github.com/CachetHQ/Cachet/issues/2903
* https://github.com/CachetHQ/Cachet/pull/2929
* https://github.com/CachetHQ/Cachet/issues/3468

The goal of this task:
Document this in the README, that this client lib is manually created + that new API endpoints must be added manually.