docs: diagrams for servant and server session

Author: iluwatar

Diff for: servant/README.md

@@ -34,6 +34,10 @@ Wikipedia says
 
 > In software engineering, the servant pattern defines an object used to offer some functionality to a group of classes without defining that functionality in each of them. A Servant is a class whose instance (or even just class) provides methods that take care of a desired service, while objects for which (or with whom) the servant does something, are taken as parameters.
 
+Sequence diagram
+
+![Servant sequence diagram](./etc/servant-sequence-diagram.png)
+
 ## Programmatic Example of Servant Pattern in Java
 
 The Servant design pattern is a behavioral design pattern that defines a class that provides some sort of service to a group of classes. This pattern is particularly useful when these classes lack some common functionality that can't be added to the superclass. The Servant class brings this common functionality to a group of classes.

Diff for: servant/etc/servant-sequence-diagram.png

@@ -34,6 +34,10 @@ Wikipedia says
 
 > A session token is a unique identifier that is generated and sent from a server to a client to identify the current interaction session. The client usually stores and sends the token as an HTTP cookie and/or sends it as a parameter in GET or POST queries. The reason to use session tokens is that the client only has to handle the identifier—all session data is stored on the server (usually in a database, to which the client does not have direct access) linked to that identifier.
 
+Sequence diagram
+
+![Server Session sequence diagram](./etc/server-session-sequence-diagram.png)
+
 ## Programmatic Example of Server Session Pattern in Java
 
 The Server Session design pattern is a behavioral design pattern that assigns the responsibility of storing session data on the server side. This pattern is particularly useful in the context of stateless protocols like HTTP where all requests are isolated events independent of previous requests.