ayab-shield: silkscreen additions for clarity

[vthunder]

It would be nice to add some additional labels to the shield’s silkscreen to help people know what needs to be plugged into where. Especially since there are multiple possible configurations (eg with headers for the 910 or 950 vs the 930), people get confused sometimes.

[VIPQualityPost]

OK, I am not sure I have fully addressed this in #48 . Maybe others can take a look and give some feedback?