docs: Add releasing notes

docs/CONTRIBUTING.md

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+# Contributing Notes
+
+## Releasing
+
+Releases are made on a reasonably regular basis by the maintainers (HashiCorp staff), using our internal tooling. The following notes are only relevant to maintainers.
+
+Release process:
+
+ 1. Update [`version/VERSION`](https://github.com/hashicorp/hc-install/blob/main/version/VERSION) to remove `-dev` suffix and set it to the intended version to be released
+ 1. Wait for [`build` workflow](https://github.com/hashicorp/hc-install/actions/workflows/build.yml) to finish
+ 1. Ensure you have the appropriate GitHub PAT set in `BOB_GITHUB_TOKEN` variable
+ 1. Set `SHA` to the corresponding (long) last commit SHA (after updating `VERSION` file) & `VERSION` to the same version
+ 1. Use `bob` to promote artifacts to **staging**
+ ```
+bob trigger-promotion \
+	--product-name=hc-install \
+	--environment=hc-install-oss \
+	--org=hashicorp \
+	--repo=hc-install \
+	--slack-channel=C01QDH3Q37W \
+	--product-version=$VERSION \
+	--sha=$SHA \
+	--branch=main \
+	staging
+ ```
+ 1. Use `bob` to promote artifacts to **production**
+ ```
+bob trigger-promotion \
+	--product-name=hc-install \
+	--environment=hc-install-oss \
+	--org=hashicorp \
+	--repo=hc-install \
+	--slack-channel=C01QDH3Q37W \
+	--product-version=$VERSION \
+	--sha=$SHA \
+	--branch=main \
+	production
+ ```