Explain application secrets (#217)

.env.production.sample

@@ -14,6 +14,8 @@ LOCAL_DOMAIN=example.com
 LOCAL_HTTPS=true
 
 # Application secrets
+# These are arbitrary strings. They should be long and cryptographically secure.
+# For Docker, `docker-compose run --rm web rake secret` will generate them.
 PAPERCLIP_SECRET=
 SECRET_KEY_BASE=