Update README to show that the site is now retired

[helenb]

Link to Ticket On Codebase

Description of Changes Made

Update the readme to make clear that this repo is retired.

How to Test

Preview the readme on this branch: https://github.com/torchbox/careers/tree/chore/update-readme

Screenshots

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MR Checklist

• [x] Add a description of your pull request and instructions for the reviewer to verify your work.
• [x] If your pull request is for a specific ticket, link to it in the description.
• [x] Stay on point and keep it small so the merge request can be easily reviewed.
• [ ] Consider adding unit tests, especially for bug fixes. If you don't, tell us why.
• [x] Tests and linting passes.
• [ ] Consider updating documentation. If you don't, tell us why.
• [ ] If relevant, list the environments / browsers in which you tested your changes.

[helenb]

@tomusher we can also make this build read-only now.

[helenb]

@tomusher that's now merged so feel free to go ahead and make this repo read-only.