Create PULL_REQUEST_TEMPLATE.md

Author: redonkulus

.github/PULL_REQUEST_TEMPLATE.md

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+Please consider the following before submitting a pull request:
+
+Guidelines for contributing: https://github.com/yahoo/pure/blob/master/CONTRIBUTING.md
+
+Example of changes on an interactive website such as the following:
+
+- http://jsbin.com/
+- http://jsfiddle.net/
+- http://codepen.io/pen/