Add documentation for window subsystem for linux (WIP)

[lrouhier]

For windows 10 user , it might be better to use windows subsystem for Linux (WSL) instead of docker especially since Docker is dropping the support on Docker toolbox. This PR add documentation for the installation of FSLeyes and SCT in this context in a separate .rst file. It contains the following instruction :

• Link for the installation of WSL
• for Xming installation (still needed for FSLeyes)
• Command to run to set up the environement to install FSL and SCT dependencies (e.g., gcc, gtk..)
• Command to install sct from 4.2.1 archive
• Command to install FSL which will install FSLeyes
• A useful note about the location of certain files.

TO DO :

• Description of windows subsystem
• Add link in the readme for Windows user to redirect them here

[lrouhier]

Yes it might be more fitting I Will open a PR there with the changes requested.