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.