doc: revive changelog
doc: revive changelog

[v1.5.1] stopped recording changes in `CHANGELOG.md`, in favor of
GitHub releases. This change wasn't recorded in CHANGELOG.md itself,
so casual inspection would suggest that the most recent version is
1.5.0 -- indeed, that's what the official Rollbar onboarding docs
show -- rather than the true most recent version 2.1.0.

Strategically, sole reliance on GitHub releases has other problems:
- Changes to the release content aren't version controlled, so
  there's no visible audit trail if someone changes the release
  notes in the GitHub UI.
- Not everyone uses or has access to the GitHub UI at the same
  time they're looking at the code. A corollary is that one should
  not have to leave the code itself to open a browser to see what
  changed in versions.
- Vendor lock-in should be avoided. We have no plans to migrate from
  GitHub, however keeping our options reduces coupling.

[v1.5.1]:https://github.com/rollbar/rollbar-php/releases/tag/v1.5.1

rollbar/rollbar-php