add support for using Nginx Proxy Manager in wis2box-stack

Nginx Proxy Manager is a popular solution for enabling SSL with automatic renewal. @erick-otenyo has been supporting wis2box-users in Africa installing NPM on their systems which requires modification in the default wis2box-stack. 

I propose to review the wis2box-stack and update documentation for users wanting to include NPM in their wis2box-stack