Add note about browsers to contributing.md

CONTRIBUTING.md

 # Contributing to Slate
 
-Thanks for contributing to Slate! Please point your pull requests at the `dev` branch, and keep your commit messages clear and informative. Thanks!
+Thanks for contributing to Slate! Please point your pull requests at the `dev` branch, and keep your commit messages clear and informative. Also, please make sure your contributions work in the most recent version of Chrome, Firefox, and IE. Thanks!
\ No newline at end of file