Update OAuth workflow diagram

Author: weavejester

README.md

@@ -135,9 +135,32 @@ and complete authentication of the user.
 
 The following image is a workflow diagram that describes the OAuth2
 authorization process for Ring-OAuth2. It should give you an overview
-of how all the different URIs interact.
+of how all the different URIs interact:
+
+```mermaid
+sequenceDiagram
+    Client->>Ring Server: GET :launch-uri
+    Ring Server-->>Client: redirect to :authorize-uri
+    Client->>Auth Server: GET :authorize-uri
+    Auth Server-->>Client: redirect to :redirect-uri
+    Client->>Ring Server: GET :redirect-uri
+    Ring Server->>Auth Server: POST :access-token-uri
+    Auth Server->>Ring Server: returns access token
+    Ring Server-->>Client: redirect to :landing-uri
+    Client->>Ring Server: GET :landing-uri
+```
+
+The following URIs are set by the authorization server and should be absolute:
+
+- `:authorize-uri`
+- `:access-token-uri`
+
+The remaining URIs are set by you, and may be any local URI you deem
+appropriate:
 
-![OAuth2 Workflow](https://github.com/weavejester/ring-oauth2/raw/master/docs/workflow.png)
+- `:launch-uri`
+- `:redirect-uri`
+- `:landing-uri`
 
 ## Contributing