Update README.md

Added the Docker Flag --name centreon so it is easier to manage the containers.  Also added /bin/bash at the end of the command so it is easy to attach to the container.
Update Dockerfile

remove additional installed php install
Merge branch 'vegasbrianc-patch-2'
Merge branch 'patch-3' of git://github.com/vegasbrianc/docker-centreon into vegasbrianc-patch-3
Merge branch 'vegasbrianc-patch-3'
Add clarification regarding run command