doc: Remove security notice from README

We had the relevant fixes in all kernels for quite some
time, so no need to call it out explicitly at the top
level.

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@gmail.com>

README.md

@@ -2,25 +2,6 @@
 
 [![CircleCI](https://circleci.com/gh/linuxkit/linuxkit.svg?style=svg)](https://circleci.com/gh/linuxkit/linuxkit)
 
-**Security Update 17/01/2018: All current LinuxKit `x86_64` kernels
-have KPTI/KAISER enabled by default. This protects against
-[Meltdown](https://meltdownattack.com/meltdown.pdf). Defences against
-[Spectre](https://spectreattack.com/spectre.pdf) are work in progress
-upstream and some have been incorporated into 4.14.14/4.9.77 onwards
-but work is still ongoing. The kernels 4.14.14/4.9.77 onwards also
-include various eBPF and KVM fixes to mitigate some aspects of
-Spectre.  The `arm64` kernels are not yet fixed. See [Greg KH's
-excellent
-blogpost](http://kroah.com/log/blog/2018/01/06/meltdown-status/) and
-this [LWN.net
-article](https://lwn.net/SubscriberLink/744287/1fc3c18173f732e7/) for
-details.**
-
-**If you run LinuxKit kernels on x86 baremetal we also strongly
-recommend to add `ucode: intel-ucode.cpio` to the kernel section of
-your YAML if you are using Intel CPUs and `linuxkit/firmware:<hash>` if
-you are using AMD CPUs.**
-
 LinuxKit, a toolkit for building custom minimal, immutable Linux distributions.
 
 - Secure defaults without compromising usability