Update README to indicate that MCAD development has ended

[dgrove-oss]

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• [ ] I've made sure the tests are passing.
• Testing Strategy
  • [ ] Unit tests
  • [ ] Manual tests
  • [ ] Testing is not required for this change

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[dgrove-oss]

As discussed with @varshaprasad96 @tardieu and @dimakis we plan to archive this repo. It is good practice to update the README accordingly before archiving.