docs(example.env): add note for new env variables (dev/prod)

need to update config files

Author: unenglishable

example.env

@@ -1,3 +1,8 @@
+# When adding a new env variable,
+# make sure to add an entry to:
+# src/config.json (dev)
+# and
+# src/docker-config.json (prod)
 VUE_APP_BACKEND_URL=http://localhost:4000
 VUE_APP_OLD_BACKEND_URL=http://localhost:8080
 VUE_APP_API_KEY=ABC123