Add document explaining how to set kernel boot options

[carlossvg]

When configuring the Linux real-time kernel it is useful to be able to configure some boot options. For example CPU isolation, nohz

One way to do this is to modify kernel cmdline => https://github.com/ros-realtime/reference-system#isolate-cpus

Acceptance criteria

• A document exists explaining how to set some real-time related boot options