Doc: Add some info to README

Fix: add clone3 and a missing DRM_IOCTL syscall

The former is needed for recent versions of some common libs, the latter
is needed for FF.

Fix w3m seccomp filter

On some systems, dynlinking makes w3m need a bunch of extra syscalls.

Fix some potentially undeclared envvars

Advertise multiple remotes on README

https://seirdy.one/2020/11/18/git-workflow-1.html

Add LICENSE

This isn't a substantial piece of software and is security related, so
it's best to keep this permissive.

BATMAN!!!! (this commit has no parents)