Create CONTRIBUTING.rst with portability guidelines

CONTRIBUTING.rst

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+
+=======================
+Contribution Guidelines
+=======================
+
+Introduction
+------------
+
+Thank you for considering to contribute to the Common Workflow Language community repository of workflow and tool descriptions.
+It is people like you that power this community effort!
+
+Following these guidelines helps to communicate that you respect the time of the community members managing and developing this
+open source project. In return, they should reciprocate that respect in addressing your issue, assessing changes, and helping
+you finalize your pull requests.
+
+We welcome enhancements and fixes to existing tool or workflow descriptions along with new contributions. We equally
+value documentation, tutorials, issue reporting, and peer support.
+
+General CWL support questions are best posted to `Biostars.org under the CWL tag <https://www.biostars.org/t/cwl>`_.
+
+------------
+Ground Rules
+------------
+
+The CWL Project is dedicated to providing a harassment-free experience for everyone, regardless of gender, gender identity and
+expression, sexual orientation, disability, physical appearance, body size, age, race, or religion. We do not tolerate
+harassment of participants in any form.
+
+This code of conduct applies to all CWL Project spaces, including the Google Group, the Gitter chat room, the Google Hangouts
+chats, both online and off. Anyone who violates this code of conduct may be sanctioned or expelled from these spaces at the
+discretion of the leadership team.
+
+For more details, see our `Code of Conduct <https://github.com/common-workflow-language/common-workflow-language/blob/master/CODE_OF_CONDUCT.md>`_
+
+----------------------
+Portability Guidelines
+----------------------
+
+This section, like the rest of this project, is a work in progress.
+
+- Put ``DockerRequirement`` and ``SoftwareRequirement`` under ``hints``, not ``requirements``
+
+-----
+Misc.
+-----
+
+Parts of this document were inspired from https://github.com/nayafia/contributing-template/blob/master/CONTRIBUTING-template.md