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.

nodejs/routes/docs.js

@@ -25,6 +25,7 @@ const values = {
 // already covered by the "deployment" entry.
 const DOCS = {
 	overview:        {title: 'Overview',        file: path.join(__dirname, '../../README.md')},
+	changelog:       {title: 'Changelog',       file: path.join(__dirname, '../../CHANGELOG.md')},
 	deployment:      {title: 'Deployment',      file: path.join(__dirname, '../../DEPLOYMENT.md')},
 	api:             {title: 'API Reference',   file: path.join(__dirname, '../../API.md')},
 	ldap:            {title: 'LDAP',            file: path.join(__dirname, '../../docs/ldap.md')},