Update Readme - correct Usage section

[chamaoskurumi]

Two little corrections:

At the moment the Usage is described as:

Usage

./init.sh [--what/-w all|r|python|julia] [--force/-f] [--help/-h]

1.) The script file is called init-env.sh, not init.sh.
2.) Before being able to run ./init-env.sh one has to change permissions via chmod +x ./init-env.sh

[mcanouil]

Thanks, fixed in #26