Added github-compatible contributing notes

CONTRIBUTING.md

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+# Contributing
+
+Any open source product is only as good as the community behind it. You can participate by sharing code, ideas, or simply helping others. No matter what your skill level is, every contribution counts.
+
+See our [high level overview](http://silverstripe.org/contributing-to-silverstripe) on silverstripe.org on how you can help out.
+
+Or, for more detailed guidance, read one of the following pages:
+
+ * [Sharing your opinion and raising issues](http://doc.silverstripe.org/framework/en/trunk/misc/contributing/issues)
+ * [Providing code, whether it's creating a feature or fixing a bug](http://doc.silverstripe.org/framework/en/trunk/misc/contributing/code)
+ * [Writing and translating documentation](http://doc.silverstripe.org/framework/en/trunk/misc/contributing/documentation)
+ * [Translating user-interface elements](http://doc.silverstripe.org/framework/en/trunk/misc/contributing/translation)