added note for mounting volumes when using WSL

[simonstey]

from https://nickjanetakis.com/blog/setting-up-docker-for-windows-and-wsl-to-work-flawlessly#ensure-volume-mounts-work ->

When using WSL, Docker for Windows expects volume paths in a format that matches this: /c/Users/user/dev/myapp. But, WSL doesn’t work like that. Instead, it uses the /mnt/c/Users/user/dev/myapp format.

If you’re running Windows 18.03 (Spring 2018) or newer you can configure WSL to mount at / instead of /mnt