Update README.md to reflect current reality

• Put UNC paths inside code block so the slashes render properly
• Move institute codes to a wiki page
• update repository layout in light of tweaks made by alt36 and cm214 last week

@cm214 I think it's appropriate that we're still insisting on approval and merge requests for changes to things like the list of default variables the project uses or the directory layout on the SMB share. But adding a new institution feels like it's something that can be done without impacting any existing set-up, which is why I've moved that to a wiki page.

N.B. the reason for doing that was to add spri in light of MR !15 (merged) , which has led me to propose https://gitlab.developers.cam.ac.uk/ch/co/ucam_wpkg/-/wikis/Packaging-Conventions#institution-specific-package-names as a hopefully non-controversial naming convention.