Add hole diameter to README

[gene-pavlovsky]

Hello,

After receiving the boards from Elecrow, I've noticed that the holes are smaller in diameter than the regular 2.54 mm pitch perfboard/protoboard. Most of the components I use do fit, an exception are 5.08 mm pitch terminal blocks - those do fit on a normal protoboard I have.

If I managed to figure it out correctly (installed KiCad for the first time), it seems that your lovely boards have holes drilled 0.8 mm. Would be nice to mention this in the README.

Thanks Gene