split readmes between demo and exercise

step1-03/exercise/README.md

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+## Exercise
+
+### Write updateRemaining function
+
+1. Get a reference to the span with the `remaining` class, and store it in a variable
+2. Use [querySelectAll](https://developer.mozilla.org/en-US/docs/Web/API/Document/querySelectorAll) to get all of the todos.
+3. Set the `innerText` of the remaining span to the length of those todos.
+4. Add updateRemaining() to addTodo
+
+### Write a clearCompleted function
+
+1. Get a reference to all of the todos with [querySelectAll](https://developer.mozilla.org/en-US/docs/Web/API/Document/querySelectorAll)
+2. Use a `for (let todo of todos)` loop to iterate over each todo
+3. Inside the for loop write an `if` statement to test if the `input` inside of the todo has a checked value of true
+   > Hint: you can use querySelector on any HTML node to find child elements within
+4. Call `todo.remove()` for any todo whos input check is true
+5. After the loop is done, run `updateRemaining()`
+6. Attach this function to the footer button
+7. Test it out!