This ticket adds README files that explain the purpose of the src and build directories.

Also, src/setup.py sdist (#21516) is asking for a README, so we make it happy too.

CC: @vbraun @jdemeyer @embray @kiwifb @dimpase @fchapoton

Component: build

Author: Matthias Koeppe

Branch/Commit: 1b45e56

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/21565