Explain use of messages

[ethanfrey]

Also that the trait name must be camel case of the package it is in.
We must import the package into the namespace of the contract using it.

Also why we need

#[contract(error=ContractError)]
#[messages(vault_api as VaultApi)]
impl VaultContract<'_> {
}

[hashedone]

Improved README already fixes this