mach-glfw: clarify the state of opengl and vulkan examples

[emidoots]

mach-glfw-opengl-example and mach-glfw-vulkan-example are not really maintained by us, they have gotten frequent contributions by others.. but since we do not really use this code at all, it tends to fall out of sync.

Occasionally someone will ask about how to get those examples working, and if they aren't a seasoned Zig developer it may be a good challenge for them to do so.

But the READMEs of both these projects, and of https://machengine.org/pkg/mach-glfw - suggests that these are up-to-date and well-maintained projects. We need to clarify that ASAP to avoid wasting people's valuable time.

[emidoots]

#1166 (comment)