contracts: Release as v3.0.0 and add reserved field to `ContractInfoOf` (#8175)

* contracts: Update README

* contracts: Add CHANGELOG.md

* contracts: Bump version to v3.0.0 and allow publish

* Typos

Co-authored-by: Andrew Jones <ascjones@gmail.com>

* Improve wording in the changelog

* contracts: Add reserved field to ContractInfoOf for future proofing

* also bump frame-benchmarking

* update lockfile

Co-authored-by: Andrew Jones <ascjones@gmail.com>
Co-authored-by: Benjamin Kampmann <ben@parity.io>