Update sequence diagrams

- Use PlantUML
- Update README

doc/sequence-diagram/advertise.plantuml

@@ -0,0 +1,15 @@
+@startuml
+Client -> Server: POST /advertise\nX-Server-Token: <JWT_TOKEN>\n\n{"Attributes": <PEER_ATTRIBUTES>, "PublicKey": <PUBLIC_KEY> }
+Server -> Server: Validate server token
+alt Success
+    Server -> Server: Create peer with public key and attributes
+    Server -> Server: Update last contact for peer ID
+    Server -> Client: 201 Created
+else Invalid server token
+    Server -> Client: 401 Unauthorized
+else Malformed advertising request
+    Server -> Client: 400 Bad request
+else Unexpected error
+    Server -> Client: 500 Server error
+end
+@enduml