docs: add immutability explanation to readme (#348)

README.md

@@ -110,6 +110,14 @@ To send a request to the Modern Treasury API, build an instance of some `Params`
 
 For example, `client.counterparties().create(...)` should be called with an instance of `CounterpartyCreateParams`, and it will return an instance of `Counterparty`.
 
+## Immutability
+
+Each class in the SDK has an associated [builder](https://blogs.oracle.com/javamagazine/post/exploring-joshua-blochs-builder-design-pattern-in-java) or factory method for constructing it.
+
+Each class is [immutable](https://docs.oracle.com/javase/tutorial/essential/concurrency/immutable.html) once constructed. If the class has an associated builder, then it has a `toBuilder()` method, which can be used to convert it back to a builder for making a modified copy.
+
+Because each class is immutable, builder modification will _never_ affect already built class instances.
+
 ## Asynchronous execution
 
 The default client is synchronous. To switch to asynchronous execution, call the `async()` method: