Add CHANGELOG and version number

Our other terraform modules adopt the following convention:

• Maintain forward-backward compatibility in major version branches (e.g. v1)
• Tag releases (e.g. 1.0.1)
• Maintain a CHANGELOG.

See, e.g, https://gitlab.developers.cam.ac.uk/uis/devops/infra/terraform/gcp-cloud-run-app#versioning

• Create a v1 branch which will track current master as we release new versions.
• Create a CHANGELOG and 1.0.1 release which adds changelog :).
• Once MR is merged, tag the current release as 1.0.1, create a GitLab release and update the v1 branch.