CONTRIBUTING.md

Summary:
https://our.intern.facebook.com/intern/dex/open-source/open-source-licenses/#a-contributing-template
Adapted https://github.com/facebook/bistro/blob/master/CONTRIBUTING.md for `Our Development Process`.
Test-driven workflow.

Reviewed By: JonCoens

Differential Revision: D4691472

fbshipit-source-id: d296c77

CONTRIBUTING.md

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+# Contributing to Duckling
+We want to make contributing to this project as easy and transparent as
+possible.
+
+## Our Development Process
+Our changes are first made in Facebook's internal repository, which provides us
+with additional lint and static analysis support. Changes are synchronized
+periodically to GitHub.
+
+## Pull Requests
+We actively welcome your pull requests.
+
+1. Fork the repo and create your branch from `master`.
+2. Write tests that describe the expected new behavior from your code changes.
+3. Regenerate the classifiers, run the test suite and see your tests failing.
+4. Add feature code to make your tests pass.
+5. Regenerate the classifiers, run the test suite and see your tests passing.
+6. If you haven't already, complete the Contributor License Agreement ("CLA").
+
+## Contributor License Agreement ("CLA")
+In order to accept your pull request, we need you to submit a CLA. You only need
+to do this once to work on any of Facebook's open source projects.
+
+Complete your CLA here: <https://code.facebook.com/cla>
+
+## Issues
+We use GitHub issues to track public bugs. Please ensure your description is
+clear and has sufficient instructions to be able to reproduce the issue.
+
+Facebook has a [bounty program](https://www.facebook.com/whitehat/) for the safe
+disclosure of security bugs. In those cases, please go through the process
+outlined on that page and do not file a public issue.
+
+## License
+By contributing to Duckling, you agree that your contributions will be licensed
+under the LICENSE file in the root directory of this source tree.