Actually add CONNTRIBUTING.md, not included in f5a5eb3

.github/CONTRIBUTING.md

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+Did you encounter an issue with using Slic3r? Fear not! This guide will help you to write a good bug report in just a few, simple steps.
+
+There is a good chance that the issue, you have encountered, is already reported. Please check the [list of reported issues](https://github.com/alexrj/Slic3r/issues) before creating a new issue report. If you find an existing issue report, feel free to add further information to that report.
+
+If possible, please include the following information when [reporting an issue](https://github.com/alexrj/Slic3r/issues/new):
+* Slic3r version (See the about dialog for the version number. If running from git, please include the git commit ID from `git rev-parse HEAD` also.)
+* Operating system type + version
+* Steps to reproduce the issue, including:
+    * Command line parameters used, if any
+    * Slic3r configuration file (Use ``Export Config...`` from the ``File`` menu - please don't export a bundle)
+    * Expected result
+    * Actual result
+    * Any error messages
+* If the issue is related to G-code generation, please include the following:
+    * STL, OBJ or AMF input file (please make sure the input file is not broken, e.g. non-manifold, before reporting a bug)
+    * a screenshot of the G-code layer with the issue (e.g. using [Pronterface](https://github.com/kliment/Printrun))
+
+Please make sure only to include one issue per report. If you encounter multiple, unrelated issues, please report them as such.
+
+Simon Tatham has written an excellent on article on [How to Report Bugs Effectively](http://www.chiark.greenend.org.uk/~sgtatham/bugs.html) which is well worth reading, although it is not specific to Slic3r.
+