Simplify methods slide (#594)

This tightens the language slightly. I also added a speaker note with
suggestions for more methods to add (if there is time).

src/basic-syntax/methods.md

@@ -1,7 +1,7 @@
 # Methods
 
-Rust has methods, they are simply functions that are associated with a particular type. The
-first argument of a method is an instance of the type it is associated with:
+Methods are functions associated with a type. The `self` argument of a method is
+an instance of the type it is associated with:
 
 ```rust,editable
 struct Rectangle {
@@ -28,3 +28,18 @@ fn main() {
 ```
 
 * We will look much more at methods in today's exercise and in tomorrow's class.
+
+<details>
+
+- Add a `Rectangle::new` constructor and call this from `main`:
+
+    ```rust,editable,compile_fail
+    fn new(width: u32, height: u32) -> Rectangle {
+        Rectangle { width, height }
+    }
+    ```
+
+- Add a `Rectangle::new_square(width: u32)` constructor to illustrate that
+  constructors can take arbitrary parameters.
+
+</details>