add documentation on merging repo histories
To complement https://gitlab.developers.cam.ac.uk/uis/devops/raven/shib-cloud/idp-frontend-container/-/issues/6, add documentation on merging git repo histories.
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To complement https://gitlab.developers.cam.ac.uk/uis/devops/raven/shib-cloud/idp-frontend-container/-/issues/6, add documentation on merging git repo histories.