Slightly more considerate devtools warning

See facebook#953.

Author: sophiebits

src/browser/ui/React.js

@@ -107,8 +107,8 @@ if (__DEV__) {
  window.top === window.self &&
  navigator.userAgent.indexOf('Chrome') > -1) {
  console.debug(
- 'Download the React DevTools for a better development experience: ' +
- 'http://fb.me/react-devtools'
+ 'If you haven\'t already, download the React DevTools for a better ' +
+ 'development experience: http://fb.me/react-devtools'
  );
 
  var expectedFeatures = [