Add README for crates that do not have one #7966
Description
Is your feature request related to a problem or challenge?
These crates do not have a README:
core
execution
physical-plan
Describe the solution you'd like
It would be good to add minimal README to each one, and update the Cargo.toml accordingly. The README should explain that each crate is a part of DataFusion and have a link to the main crate.
Describe alternatives you've considered
No response
Additional context
No response