Create and point to GLFW_C_REVISION.txt file.

This way, the next time we update to latest GLFW C source using the
steps listed at
https://github.com/go-gl/glfw3/wiki/Development#updating-glfw-c-source,
the diff of that file will be an accurate representation of the GLFW C
source change.
It will also not require manual (error prone) updating of the README.