Add information about bug with JupyterHub 3+

Author: krokicki

README.md

@@ -16,6 +16,10 @@ This package formerly included WrapSpawner and ProfilesSpawner, which provide me
 
 ## Installation
 
+> **⚠ NOTE**
+> 
+> If you are using JupyterHub 3+ then you currently must install using the `pip install -e` method, due to a bug that [will be fixed](https://github.com/jupyterhub/batchspawner/issues/277) in the next release.
+
 1. from root directory of this repo (where setup.py is), run `pip install -e .`
 
    If you don't actually need an editable version, you can simply run