Document missing OpenAPI/Swagger-Defintion/API-Spec

[andygrunwald]

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
• Update documentation about Cachet API cachethq/cachet#2903
• Generation of the API documentation cachethq/cachet#2929
• Swagger/OpenAPI .json cachethq/cachet#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.