Add CONTRIBUTING document (#113)

.github/CONTRIBUTING.md

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+# Contributing
+
+We'd love your help making `vanguard-go` better.
+
+If you'd like to add new exported APIs, please [open an issue][open-issue]
+describing your proposal — discussing API changes ahead of time makes
+pull request review much smoother. In your issue, pull request, and any other
+communications, please remember to treat your fellow contributors with
+respect!
+
+Note that you'll need to sign [Buf's Contributor License Agreement][cla]
+before we can accept any of your contributions. If necessary, a bot will remind
+you to accept the CLA when you open your pull request.
+
+## Setup
+
+[Fork][fork], then clone the repository:
+
+```bash
+git clone [email protected]:your_github_username/vanguard-go.git
+cd vanguard-go
+git remote add upstream https://github.com/connectrpc/vanguard-go.git
+git fetch upstream
+```
+
+You will need an up-to-date installation of the Go programming language.
+
+To run tests, linting, and generate code, use the default Make target.
+Make sure it succeeds when you first check out the repository:
+
+```bash
+make
+```
+
+## Making Changes
+
+Start by creating a new branch for your changes:
+
+```bash
+git checkout main
+git fetch upstream
+git rebase upstream/main
+git checkout -b cool_new_feature
+```
+
+Make your changes, then ensure that `make` still passes.
+When you're satisfied with your changes, push them to your fork.
+
+```bash
+git commit -a
+git push origin cool_new_feature
+```
+
+Then use the GitHub UI to open a pull request.
+
+At this point, you're waiting on us to review your changes. We *try* to respond
+to issues and pull requests within a few business days, and we may suggest some
+improvements or alternatives. Once your changes are approved, one of the
+project maintainers will merge them.
+
+We're much more likely to approve your changes if you:
+
+- Add tests for new functionality.
+- Write a [good commit message][commit-message].
+- Maintain backward compatibility.
+
+[fork]: https://github.com/connectrpc/vanguard-go/fork
+[open-issue]: https://github.com/connectrpc/vanguard-go/issues/new
+[cla]: https://cla-assistant.io/connectrpc/vanguard-go
+[commit-message]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
+