Document userns remapping

[sj26]

userns remapping and running privileged containers are incompatible:

docker: Error response from daemon: privileged mode is incompatible with user namespaces. You must run the container in the host namespace when running privileged mode.

It'd be nice to document that we're using userns remapping, and the privileged caveat, and that there is an option to disable it. I wasn't quite sure where and how this might belong in the README — any ideas @lox?

[lox]

I am increasingly tempted to ditch user namespace remapping.

[nitrocode]

What is the point of user namespace remapping? Why is it enabled by default ?