Add comment about manual PartialEq impl

src/geometry.rs

@@ -67,6 +67,7 @@ impl Add for Point {
     }
 }
 
+// These can be derived too, but implement them for demonstration.
 impl PartialEq for Point {
     fn eq(&self, other: &Self) -> bool {
         self.x == other.x && self.y == other.y