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.

.dockerignore

@@ -26,6 +26,7 @@ ops/proxy.service
 # at /docs (routes/docs.js) so they're readable without internet access.
 *.md
 !README.md
+!CHANGELOG.md
 !DEPLOYMENT.md
 !nodejs/api.md
 !docs/**/*.md