Add instructions on how to run on Debian

I installed brave today, and got this errors:

Log:

usr@LINUX:~$ sudo apt install brave
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:

[removed for privacy]

Use 'sudo apt autoremove' to remove them.
The following NEW packages will be installed:
brave
0 upgraded, 1 newly installed, 0 to remove and 3 not upgraded.
Need to get 97.8 MB of archives.
After this operation, 443 MB of additional disk space will be used.
Get:1 https://s3-us-west-2.amazonaws.com/brave-apt stretch/main amd64 brave amd64 0.25.2-1 [97.8 MB]
Fetched 97.8 MB in 10s (9,152 kB/s)                                                                             
Selecting previously unselected package brave.
(Reading database ... 309999 files and directories currently installed.)
Preparing to unpack .../brave_0.25.2-1_amd64.deb ...
Unpacking brave (0.25.2-1) ...
Processing triggers for mime-support (3.60) ...
Processing triggers for desktop-file-utils (0.23-1) ...
Processing triggers for bamfdaemon (0.5.3-2) ...
Rebuilding /usr/share/applications/bamf-2.index...
Processing triggers for gnome-menus (3.13.3-9) ...
Setting up brave (0.25.2-1) ...
usr@LINUX:~$ brave
[16129:16129:1104/213523.113443:FATAL:zygote_host_impl_linux.cc(116)] No usable sandbox! Update your kernel or see https://chromium.googlesource.com/chromium/src/+/master/docs/linux_suid_sandbox_development.md for more information on developing with the SUID sandbox. If you want to live dangerously and need an immediate workaround, you can try using --no-sandbox.
Trace/breakpoint trap
usr@LINUX:~$ 

Installation steps followed: https://github.com/brave/browser-laptop/blob/master/docs/linuxInstall.md

[jacobc-eth]

I would like to add on here. There should be no reason to lower the security settings of the kernel as a workaround. Here is what the Arch documentation writes about this:

Unprivileged usage is disabled by default unless the kernel.unprivileged_userns_clone sysctl 1 is set to 1 , since it greatly increases the attack surface for local privilege escalation.

https://wiki.archlinux.org/index.php/security

This seems like a significant security issue and it is very bad that many people are being told to reduce their kernel security. We need to solve the internal sandbox issue within Brave.

[diracdeltas]

We require user namespaces because Chromium’s builtin setuid sandbox is being deprecated: https://chromium.googlesource.com/chromium/src/+/HEAD/docs/linux_suid_sandbox.md

In the nearish future, Chromium linux users will also need to enable userns.

EDIT: see https://groups.google.com/a/chromium.org/forum/m/#!topic/security-dev/t5Zu0i3PbYE and https://bugs.chromium.org/p/chromium/issues/detail?id=312380 for more details