Add Warning About Semi-Transparent Printed Parts

[ampp33]

This may be semi-obvious, but we should add a warning to the README and manual about ensuring the printed parts aren't transparent or semi-transparent if you plan on using the optical based sensor, because if the printed part isn't able to fully block the sensor's light then TAP will be less accurate and in some cases unsuable.

I printed the TAP center part using a bright orange ABS filament and noticed that TAP would seldom trigger properly, trigger late, or not trigger at all. Once I swapped that part to a darker opaque based print all my issues were resolved.