docs: update contracts/README.md

[Theo6890]

Hey guys, first of all thank you very much for this library it so helpful 🙏

I do have a recommendation to complete the Layout section to explain the role of internal interfaces and regular interfaces.

Here is what I propose:

folder	layer	contents	description	example
-	'internal' interface	custom error, enum, struct, event & preferably modifier	parent of IXyz.sol	IDiamondWritableInternal.sol, IDiamondReadableInternal.sol
-	interface	function prototypes	child of internal interfaces (e.g. IRandomnessWritable is IRandomnessWritableInternal)	IDiamondFallback.sol, IDiamondReadble.sol, IDiamondWritable.sol

[ItsNickBarry]

There are some issues with the inheritance patterns that must be resolved, then we will add this type of documentation.

Specifically, this PR does not compile: solidstate-network/layerzero-client#3

The resolution might affect the recommended patterns.