add readme staging directories (dev)

README.md

 ```
 cp ./conf/pyheatpump.systemd.timer /etc/systemd/system/pyheatpump.timer
 ```
+
+## Environment
+
+Depending of which staging environment you attended to use, please check the dedicated directory in `staging`. You'll find their a README and an install file that will help you further.

staging/dev/README.md

+# Dev environment
+
+This directory is dedicated to configure a dev environment. You could find installation instruction in the INSTALL file