Add Contributing file

CONTRIBUTING.mdown

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+## Submitting issues
+
+### Read the Help page
+
+[The Help page in the docs](http://isotope.metafizzy.co/docs/help.html) covers most issues. Please look it over first before submitting an issue.
+
+### Provide an isolated example with a live URL
+
+**A live URL is required** for any bug report or personal support request. Try re-creating the issue in jsFiddle using this template:
+
++ [jsFiddle jQuery Isotope template](http://jsfiddle.net/desandro/DQydj/)
+
+### Look over the Issues Agreement
+
+I wrote an [Issues Agreement](https://github.com/desandro/issues-agreement/#readme) that covers how to submit a good issue.