Guidelines: Add "Choosing Between Structs and Classes"

Author: shiki

docs/README.md

@@ -13,3 +13,14 @@
 
 The guidelines for how Swift should be written and formatted can be found in the [Coding Style Guide](coding-style-guide.md).
 
+### Choosing Between Structures and Classes
+
+In addition to the [Apple guidelines](https://developer.apple.com/documentation/swift/choosing_between_structures_and_classes), we generally prefer to use `struct` for: 
+
+- Value types like the [Networking Models](../../../Networking/Networking/Model)
+- Stateless helpers 
+
+But consider using `class` instead if:
+
+- You need to manage mutable states. Especially if there are more than a few `mutating` functions, the `struct` becomes harder to reason about.
+- You have to set a `struct` property declaration as `var` because it has a `mutating` function. In this case, a constant (`let`) `class` property may be easier to reason about.