Add README with description of problem.

README.md

@@ -0,0 +1,16 @@
+# Problem
+
+Why does the doc test for `sub` under `impl<T> Sub for Point2<T>` work, but my
+attempt to implement `area` under `impl<T> Bounds2<T>` fails to compile with:
+
+```
+error[E0369]: binary operation `-` cannot be applied to type `Point2<T>`
+  --> src/lib.rs:87:28
+   |
+87 |         let d = self.p_max - self.p_min;
+   |                 ---------- ^ ---------- Point2<T>
+   |                 |
+   |                 Point2<T>
+   |
+   = note: an implementation of `std::ops::Sub` might be missing for `Point2<T>`
+```