Document how to release a new version (#363)

CONTRIBUTING.md

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+# Contributing
+
+## Changelog
+
+The changelog in this repo is managed manually in the `CHANGELOG.md` file.
+PRs with a notable change should include an entry in the `HEAD (Unreleased)`
+section of the file.
+
+## Releasing
+
+To release a new version of `pulumi-policy`, update the `CHANGELOG.md` file,
+moving all items from the `HEAD (Unreleased)` section to a new section with
+the new version number. Once this is merged, a new version will be published
+when a tag of the form `v*.*.*` is pushed to the repo. To push the tag,
+ask the `@release-bot` in the internal Pulumi Slack channel `#release-ops`
+to do a release, for example:
+
+```
+@release-bot release pulumi-policy minor
+```