CONTRIBUTING: add automatic hints file

(cherry picked from commit 45ba245b)

CONTRIBUTING.md

+Contributing to OPNsense
+========================
+
+Thanks for considering a pull request or issue report.  Below are a
+few hints and tips in order to make them as effective as possible.
+
+Issue reports
+-------------
+
+Issue reports can be bug reports or feature requests.  Make sure to
+search the issues before adding a new one.  It is often better to
+join ongoing discussions on similar issues than creating a new one
+as there may be workarounds or ideas available.
+
+When creating bug reports, please make sure you provide the following:
+
+* Current OPNsense version where the bug first appeared
+* The last OPNsense version where the bug did not appear
+* The exact URL of the GUI page involved (if any)
+* A list of steps to replicate the bug
+
+All issues reported will have to be triaged and prioritised.  As we
+are a small time we may not always have the time to implement and help,
+but reporting an issue may help others to fill in.
+
+Feature requests that are in line with project goals will eventually
+be added to our roadmap:
+
+https://opnsense.org/about/road-map/
+
+Feature requests beyond the scope of OPNsense may still be doable using
+the plugin framework:
+
+https://github.com/opnsense/plugins
+
+Stale issues are timed out after a few months of inactivity.
+
+And above all: stay kind and open.  :)
+
+Pull requests
+-------------
+
+When creating pull request, please heed these points:
+
+* Base your code on the latest master branch to avoid manual merges.
+* Code review may ensue in order to help shape your proposal.
+* Pull request must adhere to 2-Clause BSD licensing.
+* Explain the problem and your proposed solution.