(docs) Update README: note on session stitching

As requested by Kris Baxter

README.md

@@ -247,6 +247,14 @@ automatically prefetched. The prefetched version loaded faster.
 
 Please note: this is by no means an exhaustive benchmark of the pros and cons of in-viewport link prefetching. Just a demo of the potential improvements the approach can offer. Your own mileage may heavily vary.
 
+## Additional notes
+
+### Session Stitching
+
+Cross-origin prefetching (e.g a.com/foo.html prefetches b.com/bar.html) has a number of limitations. One such limitation is with session-stitching. b.com may expect a.com's navigation requests to include session information (e.g a temporary ID - e.g b.com/bar.html?hash=<>&timestamp=<>), where this information is used to customize the experience or log information to analytics.  If session-stitching requires a timestamp in the URL, what is prefetched and stored in the HTTP cache may not be the same as the one the user ultimately navigates to. This introduces a challenge as it can result in double prefetches. 
+
+To workaround this problem, you can consider passing along session information via the [ping attribute](https://caniuse.com/#feat=ping) (separately) so the origin can stitch a session together asynchronously.
+
 ## Related projects
 
 * Using [Gatsby](https://gatsbyjs.org)? You already get most of this for free baked in. It uses `Intersection Observer` to prefetch all of the links that are in view and provided heavy inspiration for this project.