Include information about how open source PDKs should be structured

[mithro]

It would be good if this repository included information about how the Google open source PDKs are structure and how things are named.

I have tried to create a document at https://bit.ly/open-source-pdks-naming which includes information about this stuff.

[mithro]

There is also the "Library Naming" section of the existing SkyWater-PDK at https://skywater-pdk.readthedocs.io/en/main/contents/libraries.html#library-naming