Add a browser support section to the README

Author: meh

README.md

@@ -95,6 +95,26 @@ StringScanner, Date, etc.
 * **opal** opal additions/special behaviour in the runtime/corelib
 * **cli** specs for opal lib (parser, lexer, grammar, compiler etc)
 
+## Browser support
+
+* Internet Explorer 6+
+* Firefox (Current - 1) or Current
+* Chrome (Current - 1) or Current
+* Safari 5.1+
+* Opera 12.1x or (Current - 1) or Current
+
+Any problem above browsers should be considered and reported as a bug.
+
+(Current - 1) or Current denotes that we support the current stable version of
+the browser and the version that preceded it. For example, if the current
+version of a browser is 24.x, we support the 24.x and 23.x versions.
+
+12.1x or (Current - 1) or Current denotes that we support Opera 12.1x as well
+as last 2 versions of Opera. For example, if the current Opera version is 20.x,
+we support Opera 12.1x, 19.x and 20.x but not Opera 15.x through 18.x.
+
+Cross-browser testing sponsored by [BrowserStack](http://browserstack.com).
+
 ## License
 
 (The MIT License)