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.
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.