Add instruction for IBC-rs integration

## Background

As part of #743 towards the launch of the IBC-rs documentation portal, this issue aims to document:

- **Content:** A step-by-step instruction, notes, and precautions, so developers can easily implement IBC-rs interfaces and confidently talk IBC after the integration. Here, we may take the `mock` module or `basecoin-rs` as an example implementation.
- **Target**: developers
