Made instructions for macOS more clear
Made instructions for macOS more clear

Without the additional steps an error like `error: failed to run custom build command for `gfx-backend-metal v0.3.3`` may occur.

Additional steps copied from:
https://github.com/gfx-rs/gfx/issues/2309#issuecomment-506130902
gfx-rs/gfx-rs