Add CHANGELOG.md, serve it in-app at /docs/changelog (closes theta42/theta-env#43)

GitHub Releases already carried real changelog notes per tag, but
those require internet access to view -- exactly what the /docs
route exists to avoid. CHANGELOG.md is a committed, Keep-a-Changelog
style file (backfilled from the v1.1.0/v1.1.1/v1.1.2 release notes),
linked from README and served at /docs/changelog alongside the rest
of the project's docs.

README.md

@@ -17,6 +17,8 @@ proxy serves them over TLS with auto-renewing certs and no downtime on changes.
 > the proxy and the SSO find each other without manual config.
 
 **Documentation:** [https://theta42.github.io/proxy/](https://theta42.github.io/proxy/)
+([CHANGELOG.md](CHANGELOG.md) for what changed in each release) — also
+readable from the running app itself at `/docs`, no internet access required.
 
 ## Screenshots