README: Add jQuery environment notes

Author: timmywil

README.md

@@ -11,6 +11,13 @@ In the spirit of open source software development, jQuery always encourages comm
 3. [Writing Code for jQuery Foundation Projects](http://contribute.jquery.org/code/)
 
 
+Environments in which to use jQuery
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+
+- [Browser support](http://jquery.com/browser-support/) differs between the master (2.x) branch and the 1.x-master branch. Specifically, 2.x does not support legacy browsers such as IE6-8. The jQuery team continues to provide support for legacy browsers on the 1.x-master branch. Use the latest 1.x release if support for those browsers is required. See [browser support](http://jquery.com/browser-support/) for more info.
+- To use jQuery in Node, browser extensions, and other non-browser environments, use only **2.x** releases. 1.x does not support these environments.
+
+
 What you need to build your own jQuery
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