Merge pull request #370 from sorryeh/add_contributing_file

Add a contribution file to help others contribute

CONTRIBUTING.md

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+## How to contribute to Psych
+
+Full details [here](https://bugs.ruby-lang.org/projects/ruby/wiki/HowToContribute)
+
+#### **Did you find a bug?**
+
+* **Do not open an issue if the bug is a security vulnerability
+  in Psych**, instead refer to our [security policy](https://www.ruby-lang.org/en/security/) and email [here]([email protected]).
+
+* **Ensure the bug was not already reported** by searching on ruby-core, the ruby github repo and on Psych's github repo. More info [here](https://bugs.ruby-lang.org/projects/ruby/wiki/HowToReport)
+
+* If you're unable to find an open issue addressing the problem, [open a new one](https://bugs.ruby-lang.org/). Be sure to include a **title and clear description**, as much relevant information as possible, and a **code sample** or an **executable test case** demonstrating the expected behavior that is not occurring.
+
+#### **Did you write a patch that fixes a bug?**
+
+* Open a new GitHub pull request with the patch for small fixes. Anything larger look [here](https://bugs.ruby-lang.org/projects/ruby/wiki/HowToContribute); submit a Feature.
+
+* Ensure you clearly describe the problem and solution. Include the relevant ticket/issue number if applicable.
+
+Psych is a volunteer effort. We encourage you to pitch in and [join the team](https://github.com/ruby/psych/contributors)!
+
+Thanks! :heart: :heart: :heart:
+
+The Psych Team