Also call out to the guidelines for using pkg-config dependencies first,
and reference it for other build systems as well.
Although the possibility of installing with a pkg-config file is
somewhat hidden away in the meson docs, it's been deemed less invasive
due to not distracting away from cmake. So it will have to do.
This serves as a minimal release-only way to embed json into a project.
Add meson support to this directly, to make it usable standalone as a
meson subproject.
Implements #1672
Eli Schwartz