CONTRIBUTING: add notes

CONTRIBUTING.md

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+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:
+
+* The current plugin version where the bug first appeared
+* Add the name of the plugin to the front of the subject
+* The last plugin version where the bug did not exist
+* The exact URL of the GUI page involved (if any)
+* A list of steps to replicate the bug
+* The current OPNsense version used
+
+All issues reported will have to be triaged and prioritised.  As we
+are a small team and outside contributors we may not always have the
+time to implement and help, but reporting an issue may help others
+to fill in.
+
+Plugins can be almost anything not necessary tied to our core mission,
+so do not be afraid to ask.
+
+Feature requests that relate to core components of OPNsense can be provided
+using the core repository:
+
+https://github.com/opnsense/core/issues
+
+Stale issues are timed out after several months of inactivity.
+
+And above all: stay kind and open.  :)
+
+Pull requests
+-------------
+
+When creating pull request, please heed the following:
+
+* 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