6684: Adding CONTRIBUTING.md file

CONTRIBUTING.md

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+# Contributing
+
+Thank you for considering contributing to the
+[JDK Mission Control](https://openjdk.java.net/projects/jmc) project! For information 
+about contributing to [OpenJDK](https://openjdk.java.net/) projects, which include
+JDK Mission Control, please see <https://openjdk.java.net/contribute/>.
+
+## Pull Requests
+
+The JDK Mission Control project (JMC) gladly accepts contributions in the form of pull 
+requests on [GitHub](https://github.com/openjdk/jmc/pulls/). Ensure that the commit 
+message refers to an existing JMC issue in the [JDK Bug System](https://bugs.openjdk.java.net/).
+The format is "&lt;bug id number&gt;: Commit message". For example, for a PR for issue JMC-6789, the commit 
+message should be "6789: Commit message".  
+
+## Mailing List
+
+The mailing list for JDK Mission Control is `[email protected]`. See
+<https://mail.openjdk.java.net/mailman/listinfo/jmc-dev> for instructions
+on how to subscribe of if you want to read the archives.
+
+## Issues
+
+You can find open issues to work on in the JMC project in the
+[JDK Bug System](https://bugs.openjdk.java.net/):
+<https://bugs.openjdk.java.net/secure/Dashboard.jspa?selectPageId=13519>.
+
+## Larger Contributions
+
+If you have a larger contribution in mind then we highly encourage you to first
+discuss your changes on the JMC mailing list, `[email protected]`,
+_before_ you start to write the code.
+
+## Slack
+
+The slack channel for JMC developers can be found here:
+<https://jdkmissioncontrol.slack.com/>
+
+If you're working for a company already contributing, you can sign up here:
+<https://jdkmissioncontrol.slack.com/signup>
+
+If not, ask for an invite in the jmc-dev mailing list.
+
+## Questions
+
+If you have a question or need help, please send an email to our mailing list
+`[email protected]` or stop by the JMC slack channel.
+