Add browser support section of README

(This is being moved from the server README where it doesn't really
belong.)

README.md

@@ -19,6 +19,25 @@ setting up a server.
 
   [4]: https://github.com/Zarel/Pokemon-Showdown
 
+Browser support
+------------------------------------------------------------------------
+
+Pokémon Showdown currently supports, in order of preference:
+
+ - Chrome
+ - Firefox
+ - Opera
+ - Safari 5+
+ - IE11+
+ - Chrome/Firefox/Safari for various mobile devices
+
+Pokémon Showdown is usable, but expect degraded performance and certain features not to work in:
+
+ - Safari 4+
+ - IE9+
+
+Pokémon Showdown is mostly developed on Chrome, and Chrome or the desktop client is required for certain features like dragging-and-dropping teams from PS to your computer. However, bugs reported on any supported browser will usually be fixed pretty quickly.
+
 Testing
 ------------------------------------------------------------------------