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.

[dpanter]

Just popping in to mention a snippet from the changelog in version 0.8; mangoapp vulkan layer is deleted (it was a testing project and no longer in development)

Now, the build instructions have been updated to mention Meson options so I consider it good enough.