Added link for flat head M3 philip screws in assembly guide.

[reggietheroman]

I saw some posts when I was building my lets split that the button head screws would get caught on some keycaps. I decided to look for flat / countersunk philips flat head screws that would be compatible with the M3 spacers. It took me a while to find them and I figured I would save some other people the time by including a link to it in the guide.

[nicinabox]

Thanks @reggietheroman!