add vendoring

vendor/github.com/jhillyerd/enmime/CONTRIBUTING.md

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+How to Contribute
+=================
+
+Enmime highly encourages third-party patches. There is a great deal of MIME
+encoded email out there, so it's likely you will encounter a scenario we
+haven't.
+
+### tl;dr:
+
+- Please add a unit test for your fix or feature
+- Ensure clean run of `make test lint`
+
+
+## Getting Started
+
+If you anticipate your issue requiring a large patch, please first submit a
+GitHub issue describing the problem or feature. Attach an email that illustrates
+the scenario you are trying to improve if possible. You are also encouraged to
+outline the process you would like to use to resolve the issue. I will attempt
+to provide validation and/or guidance on your suggested approach.
+
+
+## Making Changes
+
+Create a topic branch based on our `master` branch.
+
+1. Make commits of logical units.
+2. Add unit tests to exercise your changes.
+3. **Scrub personally identifying information** from test case emails, and
+   keep attachments short.
+4. Ensure the code builds and tests with `make test`
+5. Run the updated code through `make lint`
+
+
+## Thanks
+
+Thank you for contributing to enmime!