feat: api authentication initial

docs/server-api.md

@@ -2,4 +2,15 @@
 title: Server API
 ---
 
-TODO: describe API usage
+### Authentication
+
+The authentication is implemented via cookies. In order to obtain a valid cookie, one must simply copy the `auth_token` one from the browser's console and use to for desired API calls.
+
+![cookie](/img/content/server-api/cookie.png)
+
+For example, in order to update a given player's skill, one can execute the following command:
+
+```bash
+$ curl -b "auth_token=<redacted>" -X PUT -H "Content-Type: application/json" -d '{"scout":4,"soldier":6,"demoman":3,"medic":2}' https://api.tf2pickup.eu/players/76561199195756652/skill
+{"scout":4,"soldier":6,"demoman":3,"medic":2}
+```