Add CONTRIBUTING.md

CONTRIBUTING.md

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+## Submitting issues
+
+If you have questions about how to install or use ownCloud, please direct these to the [mailing list][mailinglist] or our [forum][forum]. We are also available on [IRC][irc].
+
+### Short version
+
+ * The [**issue template can be found here**][template]. Please always use the issue template when reporting issues.
+
+### Guidelines
+* Please search the existing issues first, it's likely that your issue was already reported or even fixed.
+  - Go to one of the repositories, click "issues" and type any word in the top search/command bar.
+  - You can also filter by appending e. g. "state:open" to the search string.
+  - More info on [search syntax within github](https://help.github.com/articles/searching-issues)
+* This repository ([contacts](https://github.com/owncloud/contacts/issues)) is *only* for issues within the ownCloud contacts code.
+* __SECURITY__: Report any potential security bug to security@owncloud.com following our [security policy](https://owncloud.org/security/) instead of filing an issue in our bug tracker
+* Report the issue using our [template][template], it includes all the information we need to track down the issue.
+
+Help us to maximize the effort we can spend fixing issues and adding new features, by not reporting duplicate issues.
+
+[template]: https://raw.github.com/owncloud/core/master/issue_template.md
+[mailinglist]: https://mailman.owncloud.org/mailman/listinfo/owncloud
+[forum]: https://forum.owncloud.org/
+[irc]: https://webchat.freenode.net/?channels=owncloud&uio=d4
+
+### Contribute Code and translations
+Please check [core's contribution guidelines](https://github.com/owncloud/core/blob/master/CONTRIBUTING.md) for further information about contributing code and translations.