Update README with es6 notice

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 A lightweight JavaScript date library for parsing, validating, manipulating, and formatting dates.
 
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+**Important notice**: Moment is undergoing major refactoring for version
+**2.10**, that would result in ES6 code that is transpiled to ES5 for
+different environments: node, browser global, AMD, various build/packaging
+systems.
+
+You might be required to rewrite your pull request on top once we merge it in.
+
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 ## [Documentation](http://momentjs.com/docs/)
 
 ## Upgrading to 2.0.0