Write documentation for new canvas module

wgpu/src/widget/canvas/drawable.rs

@@ -0,0 +1,12 @@
+use crate::canvas::Frame;
+
+/// A type that can be drawn on a [`Frame`].
+///
+/// [`Frame`]: struct.Frame.html
+pub trait Drawable {
+    /// Draws the [`Drawable`] on the given [`Frame`].
+    ///
+    /// [`Drawable`]: trait.Drawable.html
+    /// [`Frame`]: struct.Frame.html
+    fn draw(&self, frame: &mut Frame);
+}