Add a guide to contributing

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+Filing a bug report? Please include the following:
+1. Link to a page which demonstrates the problem, preferably a jsfiddle (use [this jsfiddle](http://dygraphs.com/fiddle) as a template)
+2. Browser and Operating System that exhibit the problem
+
+It also helps if you include the non-compacted version of the JS on your
+page. For instance, instead of doing this:
+
+```html
+<script type="text/javascript" src="dygraph-combined.js"></script>
+```
+
+do this:
+
+```html
+<script type="text/javascript" src="dygraph-dev.js"></script>
+```
+
+This makes error messages and debugging simpler. The jsfiddle does this automatically.
+
+Sending a Pull Request? See our [guide to making dygraphs contributions](http://dygraphs.com/changes.html).