Add mybinder badge

I suggest to add a mybinder badge to the template project (as is the case for the eossr library), I tested it and adding

[![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/https%3A%2F%2Fgitlab.in2p3.fr%2Fescape2020%2Fwp3%2Ftemplate_project_escape/HEAD)

to the README.md works correctly.