Add documentation for how to integrate external light clients to a chain

[colin-axner]

Summary

We should have documentation explaining how external light clients can be integrated into a chain

---

For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged/assigned

[colin-axner]

Consider applying the suggestion here #2825 (comment)

We could move all the ibc registration to the bottom of the stack and then add lines of comments for registering our client types

ModuleBasics = module.NewBasicManager(
    ...
    ibc.AppModuleBasic{},
    
    // register light clients on IBC
    ibctm.AppModuleBasic{},
    solomachine.AppModuleBasic{},
}