More In-Depth Component Documentation

[Mytherin]

I suggest adding more in-depth documentation for the different components in the form of a README.md file inside the different subdirectories. I.e. there could be a README.md inside the src/transaction directory explaining the MVCC, one in the execution directory explaining the execution model, etc... just to make it easier to understand the different components and how they relate to each other.

[hannes]

Ha