docs: Adds hardware requirements to Install Guide
Puts the nested virt/bare metal requirement in the top line
of the Install Guide and references the Kata hardware
check.
# Kata Containers installation user guides
+Kata Containers requires nested virtualization or bare metal.
+See the [hardware requirements](https://github.com/kata-containers/runtime/blob/master/README.md#hardware-requirements) to see if your system is capable of running Kata Containers.
+
Select your preferred distribution:
* [CentOS](https://github.com/kata-containers/documentation/blob/master/install/centos-installation-guide.md)
