Include information about how the open source PDKs are versioned

[mithro]

It would be good for this repository to document how we are versioning the open source PDKs.

The SkyWater PDK documents the version number information at https://skywater-pdk.readthedocs.io/en/main/versioning.html#version-number-format

[mohanad0mohamed]

@mithro I think this issue should be closed as it's already merged with requested fixes.