Mention Safari ≥ 12.1 working without polyfills

Safari 12.1 shipped Intersection Observer.

README.md

@@ -214,8 +214,8 @@ quicklink({
 
 The prefetching provided by `quicklink` can be viewed as a [progressive enhancement](https://www.smashingmagazine.com/2009/04/progressive-enhancement-what-it-is-and-how-to-use-it/). Cross-browser support is as follows:
 
-* Without polyfills: Chrome, Firefox, Edge, Opera, Android Browser, Samsung Internet.
-* With [Intersection Observer polyfill](https://github.com/w3c/IntersectionObserver/tree/master/polyfill) ~6KB gzipped/minified: Safari, IE11
+* Without polyfills: Chrome, Safari ≥ 12.1, Firefox, Edge, Opera, Android Browser, Samsung Internet.
+* With [Intersection Observer polyfill](https://github.com/w3c/IntersectionObserver/tree/master/polyfill) ~6KB gzipped/minified: Safari ≤ 12.0, IE11
 * With the above and a [Set()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set) and [Array.from](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/from) polyfill: IE9 and IE10. [Core.js](https://github.com/zloirock/core-js) provides both `Set()` and `Array.from()` shims. Projects like [es6-shim](https://github.com/paulmillr/es6-shim/blob/master/README.md) are an alternative you can consider.
 
 Certain features have layered support: