NanoBSD: add how to obtain the source code

There is a request in the bugzilla report to introduce how to obtain the
source code for building NanoBSD. I added more lines to the patch
provided by the original reporter.

PR: 274709
Reviewed by: mhorne
Co-Authored-By: gaelan@fastmail.fm
Differential Revision: https://reviews.freebsd.org/D42743