Add CONTRIBUTING.md

* fixes #16

CONTRIBUTING.md

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+# Contributing to Reactivated
+
+As an open source project, Reactivated welcomes contributions of many forms.
+
+Examples of contributions include:
+
+* Code patches
+* Documentation improvements
+* Bug reports and patch reviews
+
+Philosophy guidelines are available online at:
+
+https://www.reactivated.io/documentation/philosophy-goals/
+
+Warning: non-trivial pull requests (anything more than fixing a typo) without discussion will be closed! Please open a discussion to suggest large changes.
+
+https://github.com/silviogutierrez/reactivated/discussions