Add a post- spec release checklist

[Julian]

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There are a few tasks that need doing just after a new version of the specification has been released, or just before:

• [ ] Copy the tests/draft-next and output-tests/draft-next directories to new directories that reflect the new version (copy, not move, as we immediately will start using draft-next for the next draft)
• [ ] Change the URIs of tests using $schema in the copied directories to the "official" new URI
• [ ] Update the latest directory symlink to point to the copied directory
• [ ] Update the READMEs for the above
  • [ ] README in root directory
  • [ ] README in output-tests directory
• [ ] Update the cross-draft tests
• [ ] Consider updating the test suite schemas (test-schema.json and output-test-schema.json)

We can/should consider automating some of the above (perhaps in the bin/jsonschema_suite tool).