Update README.md

add notes

examples/leantime/README.md

@@ -1,3 +1,9 @@
 # References
 
 - https://github.com/Leantime/docker-leantime
+
+# Notes
+
+Ensure to define the variable `LEAN_APP_URL` in the `.env` file. 
+
+Otherwise, you may get errors (CSS/JS not properly loaded). Especially if you use a reverse proxy in front.