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:

* Initial Issue Tracker Ticket: https://bugs.chromium.org/p/gerrit/issues/detail?id=11401&q=swagger&can=2
* ESC Meeting notes: https://www.gerritcodereview.com/2019-12-10-esc-minutes.html
* ESC Meeting notes: https://www.gerritcodereview.com/2020-01-21-esc-minutes.html

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.