Add contributing guide

CONTRIBUTING.md

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+How to Contribute
+=================
+
+Inbucket encourages third-party patches. It's valuable to know how other
+developers are using the product.
+
+
+## Getting Started
+
+If you anticipate your issue requiring a large patch, please first submit a
+GitHub issue describing the problem or feature. 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
+
+Inbucket uses [git-flow] with default options.  If you have git-flow installed,
+you can run `git flow feature start <topic branch name>`.
+
+Without git-flow, create a topic branch from where you want to base your work:
+  - This is usually the `develop` branch, example command:
+    `git checkout origin/develop -b <topic branch name>`
+  - Only target the `master` branch if the issue is already resolved in
+    `develop`.
+
+Once you are on your topic branch:
+
+1. Make commits of logical units.
+2. Add unit tests to exercise your changes.
+3. Run the updated code through `go fmt` and `go vet`.
+4. Ensure the code builds and tests with the following commands:
+  - `go clean ./...`
+  - `go build ./...`
+  - `go test ./...`
+
+
+## Thanks
+
+Thank you for contributing to Inbucket!
+
+[git-flow]: https://github.com/nvie/gitflow