chore(docs): add migration notice

README.md

@@ -1,3 +1,5 @@
+> **ATTENTION:** As part of a major update, we will be replacing the existing `phosphor-react` package with `@phosphor-icons/react`. We recommend using this new version, as it has improved performance and significantly smaller bundle size. No APIs have been changed, so drop-in replacement should be straightforward. The legacy package will continue to recieve maintenance, but will not be updated with new icons upstream. [Take me to the legacy version ➜](https://github.com/phosphor-icons/react/tree/legacy)
+
 <img src="/meta/phosphor-mark-tight-yellow.png" width="128" align="right" />
 
 # phosphor-react
@@ -77,7 +79,7 @@ const App = () => {
       <div>
         <Horse /> {/* I'm lime-green, 32px, and bold! */}
         <Heart /> {/* Me too! */}
-        <Cube />  {/* Me three :) */}
+        <Cube /> {/* Me three :) */}
       </div>
     </IconContext.Provider>
   );
@@ -94,7 +96,7 @@ You may create multiple Contexts for styling icons differently in separate regio
 
 <img src="/meta/cube-rotate.svg" width="128" align="right" />
 
-Components can accept arbitrary SVG elements as children, so long as they are valid children of the `<svg>` element. This can be used to modify an icon with background layers or shapes, filters, animations and more. The children will be placed *below* the normal icon contents.
+Components can accept arbitrary SVG elements as children, so long as they are valid children of the `<svg>` element. This can be used to modify an icon with background layers or shapes, filters, animations and more. The children will be placed _below_ the normal icon contents.
 
 The following will cause the Cube icon to rotate and pulse: