I correct the mistakes of grammer in README.md. 'update' word is an countable noun.

README.md

@@ -113,4 +113,4 @@ Please ask us if you want to distribute your code under another license.
 See the [contribution guidelines](https://github.com/mruby/mruby/blob/master/CONTRIBUTING.md)
 then send a pull request to <http://github.com/mruby/mruby>.  We consider you have granted
 non-exclusive right to your contributed code under MIT license.  If you want to be named
-as one of mruby developers, include update to the AUTHORS file in your pull request.
+as one of mruby developers, please include an update to the AUTHORS file in your pull request.