Enable tracking (#813)

* Enable tracking

* Add documentation

CONTRIBUTING.md

@@ -35,6 +35,24 @@ The GraphQL API can be queried with a POST request to `/api/graphql`. There is a
 
 The REST API can be queried at `/api/v1`. An interactive Swagger API explorer is available at `/api/swagger-ui`.
 
+### Tracker client authorization
+#### OAuth
+Since the url of a Suwayomi-Server is not known, it is not possible to redirect directly to the client.<br/>
+Thus, to provide tracker support via oauth, the tracker clients redirect to the [suwayomi website](https://suwayomi.org/)
+and there the actual redirection to the client takes place.
+
+When implementing the login process in your client you have to make sure to follow some preconditions:
+
+To be able to redirect to the client you have to attach a `state` object to the query of the auth url
+- this `state` object has to have a `redirectUrl` which points to the client route at which you want to handle the auth result
+- besides the `redirectUrl` you can pass any information you require to handle the result (e.g. the server `id` of the tracker client)
+- example URL for AniList: `https://anilist.co/api/v2/oauth/authorize?client_id=ID&response_type=token&state={ redirectUrl: "http://localhost:4567/handle/oauth/result", trackerId: 1, anyOtherInfo: "your client requires" }`
+
+Once the permission has been granted, you will get redirected to the client at the provided route (`redirectUrl`).<br/>
+- Example URL (decoded) for AniList: `http://localhost:4567/tracker/login?access_token=TOKEN&token_type=Bearer&expires_in=31622400&state={ redirectUrl: "http://localhost:4567/handle/oauth/result", trackerId: 1, anyOtherInfo: "your client requires" }`).<br/>
+
+Finally, to finish the login process, you just have to pass this URL to the server as the `callbackUrl`.
+
 ## Why a web app?
 This structure is chosen to
 - Achieve the maximum multi-platform-ness