README: Reduce the textdump; describe the project

Rework the README to make it a little easier for new users. This is the
first file many will see when persuing the FreeBSD source code so

• remove some of the text describes how to build. This is better covered

in the linked documentation.

• add a small blurb for what FreeBSD is. Some users might find this

document through features such as github search so they may not even
know what the project is

generally, gear this file for the new, accidental, or casual user rather
than towards someone seeking fuller documentation.