Fix README has a link to a non-existent changelog

[joseph-robertson]

Pull Request Description

Closes #1296. In the top level README, point to RTD Changelogs page instead of the old (removed) CHANGELOG.md file.

Checklist

Not all may apply:

• Tests (and test files) have been updated
• Documentation has been updated
  • If related to resstock-estimation, checklist includes data dictionary, source report, options saturation, options_lookup.
• Add to the changelog_dev.rst file
• openstudio tasks.rb update_measures has been run
• No unexpected regression test changes on CI (checked comparison artifacts)