Link to roadmap is available in README

[nialloriordan]

What

Update the broken markdown link for the roadmap in the README

How

Replace the existing link to a markdown file with an external link to the roadmap.

Community member or Airbyter

• [x] Community member? Grant edit access to maintainers (instructions)
• [x] Code reviews completed
• [x] PR name follows PR naming conventions

Airbyter

If this is a community PR, the Airbyte engineer reviewing this PR is responsible for the below items.

• [ ] Create a non-forked branch based on this PR and test the below items on it
• [ ] Build is successful
• [ ] If new credentials are required for use in CI, add them to GSM. Instructions.
• [ ] /test connector=connectors/<name> command is passing
• [ ] New Connector version released on Dockerhub by running the /publish command described here
• [ ] After the connector is published, connector added to connector index as described here
• [ ] Seed specs have been re-generated by building the platform and committing the changes to the seed spec files, as described here

[CLAassistant]

All committers have signed the CLA.