Add hint how to enable always-on feature

It's not reasonnably possible to set android's system setting 'alsways-on vpn' from within Bitmask.

It would be nice to get a hint how to get to the system settings. It should be also programmatically possible to open the android system settings activity containing the always-on vpn option from within the app (but this can get hacky, as the vendors might change the position of settings options and the options might differ between the android API levels)