Document how to perform format mappings in D6/D7 upgrade

Here are some examples where the migration definition file has been edited, but I think it would be better to use hook_migration_plugins_alter if you need to apply this change everywhere

In my example, the D7 source site's text formats weren't migrated properly when the site was upgrade to d7, so machine names are in the d6 format.

Node body field:

process:
  body:
    -   
      plugin: sub_process
      source: body
      process:
        value: value
        format:
          -   
            plugin: static_map
            bypass: true
            source: format
            map:
              '1': 'basic_html'
              '2': 'full_html'
              '3': 'plain_text'
              '4': 'plain_text'
            default_value: 'basic_html'

Term description field:

process:
  description/value:
    -       
      plugin: get 
      source: description
  description/format:
    -       
      plugin: static_map
      source: format
      map:
        '1': 'basic_html'
        '2': 'full_html'
      default_value: 'basic_html'

Or a simple default value also works:

process:
  description/format:
    -   
      plugin: default_value
      default_value: 'basic_html'