Explain how to report issues

Author: WhyNotHugo

CONTRIBUTING.md

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+# Contributing to python-barcode
+
+## Bug reports
+
+Please make sure that you take the following steps when reporting issues:
+
+Ensure that you are using the latest version. You bug may already have been
+fixed.
+
+Search existing issues to ensure that it was not previously included. Be sure to
+search closed issues too, your issue may have been fixed recently.
+
+In case that the generated output is not correct, please mention:
+
+- What input did you provide?
+- What output did you get?
+- What output do you think you should have obtained?
+
+In case of crashes, please copy the full stack trace and **at least** the line
+from your code that reproduces the crash.
+
+Trying to understand and debug any error is a lot easier if you provide a small
+portion of code that I can run myself to reproduce the error.