Add release instructions to CONTRIBUTING.md

Resolves open-telemetry#112

CONTRIBUTING.md

@@ -7,3 +7,27 @@ for general information about the project.
 ## Prerequisites
 
 - Docker
+
+## Creating a new PR
+
+If there is a `CHANGELOG.md` file in the component you are updating,
+please make sure to add an entry for your change in the "Unreleased" section.
+
+## Release instructions for maintainers
+
+1. Add new desired version number in all `CHANGELOG.md` files (instead of "Unreleased") and ensure no (relevant) entries are missing
+   - This currently only applies to [semantic-conventions/CHANGELOG.md](./semantic-conventions/CHANGELOG.md)
+1. Create the release at <https://github.com/open-telemetry/build-tools/releases/new>
+   1. Tag: `v0.xx.y`
+   1. Title: `Release version 0.xx.y`
+   1. Click on _Generate release notes_
+   1. Structure release notes with the headings used in former release notes (since one release applies to multiple distinct components)
+   1. Verify that the release looks like expected and hit _Publish release_
+1. Verify the release
+   - Ensure all workflows in <https://github.com/open-telemetry/build-tools/actions> succeeded (branch = the new version tag)
+   - Ensure all images were pushed to <https://hub.docker.com/u/otel> as expected. Currently these are:
+     - <https://hub.docker.com/r/otel/semconvgen/tags>
+     - <https://hub.docker.com/r/otel/build-tool-schemas/tags>
+     - <https://hub.docker.com/r/otel/build-protobuf/tags>
+     - <https://hub.docker.com/r/otel/cpp_format_tools/tags>
+1. Update the respective version references in the <https://github.com/open-telemetry/opentelemetry-specification> repository, if needed