Update dcumentation for separate /usr
 

Hi guys,

The udev package (version 181 and higher) will not support separate /usr
partitions without using an initramfs. Because of this, we will need to
update our documents to inform the users about this:
- inform them that using a separate /usr requires an initramfs
- tell them how to build their own initramfs

I've opened a tracker bug [1] so that we can track issues related with it,
as well as major changes that still need to be done (if any). It also allows
the udev maintainers to block on this bug before stabilizing.

Wkr,
Sven Vermeulen

[1] https://bugs.gentoo.org/show_bug.cgi?id=407959