docs: producer-consumer and promise diagrams

promise/README.md

@@ -37,6 +37,10 @@ Wikipedia says
 
 > In computer science, future, promise, delay, and deferred refer to constructs used for synchronizing program execution in some concurrent programming languages. They describe an object that acts as a proxy for a result that is initially unknown, usually because the computation of its value is not yet complete.
 
+Sequence diagram
+
+![Promise sequence diagram](./etc/promise-sequence-diagram.png)
+
 ## Programmatic Example of Promise Pattern in Java
 
 The Promise design pattern is a software design pattern that's often used in concurrent programming to handle asynchronous operations. It represents a proxy for a value not necessarily known when the promise is created. It allows you to associate handlers with an asynchronous action's eventual success value or failure reason.