Add migration doc for mempool config refactor

Follow-on task for [https://github.com/cosmos/evm/pull/496](https://github.com/cosmos/evm/pull/496). We need to document the migration for the mempool initialization for those that do not use the default settings. Todo for the migration from v0.4 to v0.5.