Update README with performance improvement note

Clarify performance improvement details for small polygons.

Author: olly-writes-code

README.md

@@ -5,7 +5,7 @@ Python bindings for [Clipper2](https://github.com/AngusJohnson/Clipper2/tree/mai
 ### Why does this exists?
 Clipper2 is now alive and kicking. It should be preferred vs the old Clipper library (see docs [here](https://www.angusj.com/clipper2/Docs/Overview.htm)).
 
-While one could leverage [Shapely](https://shapely.readthedocs.io/en/stable/) for some 2D line and shape operations, it's performance is limited. Using this library you can get a ~80% performance improvement for operations like union and difference.
+While one could leverage [Shapely](https://shapely.readthedocs.io/en/stable/) for some 2D line and shape operations, it's performance is limited. Using this library you can get a ~80% performance improvement for operations like union and difference (for small polygons).
 
 ### ⚠️ Warning!