Update README.md

Please, see [Safari 10 release notes](https://developer.apple.com/library/prerelease/content/releasenotes/General/WhatsNewInSafari/Articles/Safari_10_0.html)

README.md

@@ -29,7 +29,7 @@ copy('Text', {
 
 # [Browser support](http://caniuse.com/#feat=document-execcommand)
 
-Works everywhere where `prompt`* is available. Works best (i.e. without additional keystrokes) in Chrome, FF and, supposedly, IE/Edge.
+Works everywhere where `prompt`* is available. Works best (i.e. without additional keystrokes) in Chrome, FF, Safari 10+, and, supposedly, IE/Edge.
 
 Note: **does not work on some older iOS devices.**  
 `*` – even though **Safari 8** has `prompt`, you cannot specify prefilled content for prompt modal – thus it **doesn't work** as expected.