Adding CONTRIBUTING.md

Author: jisedlac

CONTRIBUTING.md

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+# Contributing to VisualVM
+
+*Copyright © 2017, 2020, Oracle Corporation and/or its affiliates. All rights reserved. GNU General Public License v2.0 with Classpath Exception.*
+
+**Pull requests are currently not being accepted for the VisualVM project.**
+
+Please let us know your ideas, missing features, or bugs found. Either [file a RFE/bug](https://github.com/oracle/visualvm/issues/new/choose) or [leave us a message](https://visualvm.github.io/feedback.html).