Add design file index + PDF export of schematics / board

[Kartoffel]

It's a bit difficult to navigate the PCB design files, especially the 'Sensor Boards' folder. For example, there are three different motion sensor boards and the differences / working statuses are not clear.
Could you add a small markdown index file listing filenames and short descriptions?

Second, my version of EAGLE CAD is too old to open most of the design files. It would be really helpful if you could add PDF exports of the schematic and board design files.
If you want to avoid cluttering the repository, you could consider bundling them in a release (#136)

Thanks for your consideration!