Add CONTRIBUTING.md guide.

CONTRIBUTING.md

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+
+# Contributing
+
+## Code of Conduct
+
+Please see `./CODE_OF_CONDUCT.md`.
+
+
+## Release workflow
+
+To create a new release:
+
+- Submit a release PR targeting the `master` branch:
+  - Bumps the version in `package.json`.
+  - Run `npm install` to update the `package-lock.json`.
+  - The commit message should be of the form "Release vx.y.z"
+  - The title of the release PR should be of the form "Release vx.y.z"
+
+- Once the PR is merged, create a new release:
+  - Go the GitHub repo, and navigate to ["Releases"](https://github.com/fortanix/openapi-to-effect/releases).
+  - Click ["Draft a new release"](https://github.com/fortanix/openapi-to-effect/releases/new).
+  - Under "Choose a new tag", create a new tag of the form `vx.y.z`.
+  - The name of the release should be of the form `vx.y.z`.
+  - Write the release notes.
+  - If the version is a pre-release, mark it as such.
+  - Hit "Publish the release".
+
+- Once the release has been created, a GitHub Actions workflow will automatically run to publish this release to npm.