Add bash as a requirement to launch the playbook.

As a developer, when I connect on a linux server, I use zsh by default as an habit. But I face issue running-it with zsh and by looking at the code in ansible, it seems the tasks are bash oriented.

My problem starting at the installation of pyenv, and this is exactly when the mention of bash begins.

My request is just to precise this requirement in the Readme.md before running the playbook to avoid this confusion in the next iteration.