Add the `description` field in the toolbox doc to describe the function in the help listing

I verified through a test that if you pass the `description` object when creating a new command, it appears in the help command listing, it would be nice to include this in the documentation!

Example: 

```
module.exports = {
  name: 'teste',
  alias: ['t'],
  description: 'teste', // HERE
  run: async (toolbox: GluegunToolbox) => {
    
  }
}
```
