README: Document non-bootstrap build instructions first

They are preferred in the now-common case that cmake is already available.

I started doing it the slow, hard way before I realized I didn't have to bootstrap. Then a colleague went to build my branch of cmake and did the same thing, but never realized they didn't have to bootstrap. They were still waiting for an executable much later when I set them straight.

Topic-rename: readme-build-with-cmake-first