The current instructions cannot be used on e.g. Ubuntu 20.04.

This commit reformats the steps to be more natural to read when
first getting started building SONiC.

* Add SONIC_BUILD_JOBS=$(nproc) to usage guide
* Explain disk usage w.r.t. SONIC_BUILD_JOBS
* Set BUILD JOBS to 4

More than 4 can cause race issues right now, and setting to nproc
might be too much as every job spawns multiple threads.

Signed-off-by: Christian Svensson <blue@cmd.nu>