:memo: Update example readmes.

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 ![](./header.png)
 
 # 03 Controlled Inputs
+
+The most common way to handle inputs and other state-holding elements is in a
+_controlled_ way. This means your app's model is the source of truth for that
+element's state, and you update that state based on user input or other events.
+
+This example shows what that means in practice. For any controlled input we need
+two things:
+
+- A field in our model (or a function to derive a value from the model) to use
+  as the input's `value` attribute.
+
+- A message variant to handle input events and update the model.
+
+```gleam
+ui.input([
+  // Input's value is fixed to the model's `value` field
+  attribute.value(model.value),
+  // Whenever the input changes, we send a `GotInput` message with the new value
+  event.on_input(GotInput)
+])
+```
+
+## Why is this beneficial?
+
+Central to Lustre's architecture is the idea that your model is the single source
+of truth for your application's UI. This opens up the door to things like serialising
+progam state to load in the future, time-travel debugging, and rehydrating your
+app's state from a server.
+
+It also gives you tighter control of when and how to update your UI in response
+to user input. In this example, we only update the model when the new input
+value is less than 10 characters long.
+
+```gleam
+case msg {
+  GotInput(value) -> {
+    let length = string.length(value)
+
+    case length <= model.max {
+      True -> Model(..model, value: value, length: length)
+      False -> model
+    }
+  }
+
+  ...
+```