Add a note about when to load the library (#305)

* Add a note about when to load the library

* Update README.md

Co-authored-by: Philip Walton <philipwalton@users.noreply.github.com>

Co-authored-by: Philip Walton <philipwalton@users.noreply.github.com>

README.md

@@ -51,6 +51,10 @@ The library supports all of the [Core Web Vitals](https://web.dev/vitals/#core-w
 
 <a name="import-web-vitals-from-npm"><a>
 
+The `web-vitals` library uses the `buffered` flag for [PerformanceObserver](https://developer.mozilla.org/en-US/docs/Web/API/PerformanceObserver/observe), allowing it to access performance entries that occurred before the library was loaded.
+
+This means you do not need to load this library early in order to get accurate performance data. In general, this library should be deferred until after other user-impacting code has loaded.
+
 ### From npm
 
 You can install this library from npm by running: