message: Add warning note about breaking changes in master
diff: This repo provides a useful functionality to build smart contracts that
  are compatible with Cosmos SDK runtime, [currently being developed](https://github.com/cosmwasm/cosmos-sdk/issues).
  +**Warning** Most likely you want to check out `v0.5.2` tag, the stable release referred to in the [documention](https://www.cosmwasm.com).
  +Master branch is undergoing some breaking API changes in preparation for a `v0.6.0` release, which should
  +provide a much more stable, future-proof API.
  +
  ## Overview
  This crate provides the bindings and all imports needed to build a smart contract.
  