[Documentation]: Add a README to errors, helping clarify it vs types/errors

[ValarDragon]

Summary

Can there be a README.md added to errors?

It was unclear to me until I looked through the source code of what the role of cosmossdk.io/errors was, vs github.com/cosmos-sdk/cosmos/types/errors , and why I had to import both

The Readme could even just point to https://pkg.go.dev/cosmossdk.io/errors , which does a great explanation.

( Though it is still unclear to me if types/errors is planned as a long term for defining the common error types, and why we don't move these common singletons into cosmossdk.io )