Add basically API documentation

docs/api/README.md

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+# TimeLimit Server API documentation
+
+The client communicates with the server using HTTP(S) requests
+and a socket.io websocket connection.
+
+All requests bodies (if any) and responses are encoded as JSON
+if not said otherwise.
+
+## /admin
+
+This endpoint is for the server administrator.
+Its interface is described at [admin.md](./admin.md).
+
+## /auth
+
+This endpoint is used for the user authentication.
+Its interface is described at [auth.md](./auth.md).
+
+## /child
+
+This endpoint is used for by devices which are used by a child.
+Its interface is described at [child.md](./child.md).
+
+## /parent
+
+This endpoint is used by devices which are used by a parent.
+Its interface is described at [parent.md](./parent.md).
+
+## /purchase
+
+This endpoint is used for handling purchases from the client.
+Its interface is described at [purchase.md](./purchase.md).
+
+## /sync
+
+This endpoint is used by clients for syncing.
+Its interface is described at [sync.md](./sync.md).
+
+## GET /time
+
+This endpoint returns the current time of the server
+as a Unix timestamp in milliseconds. This does not need
+any authentication.
+
+The response is a object with the property ``ms`` whose
+value is the timestamp as number.
+
+Example response: ``{"ms":1578311020747}``
+
+## Websocket
+
+The websocket is used for real time notifications from the server
+to the client. The protocol is described at [websocket.md](./websocket.md).