Include build instructions for mangoapp in README.md

[dpanter]

Built mangohud/app today (6a68814) on Siduction (Debian sid base) with meson build -Dmangoapp=true -Dmangoapp_layer=true

Both seem to work properly. Would be nice if the build instructions included mangoapp. To be clear, mangoapp did not build with the default meson build command, even though the last commit seems to indicate this should happen.