Sum type parsing

Hi, I wanted to use a sum type in my command line parsing, and I came up with this:

```haskell
data FrontendLanguage = Typescript | Swift deriving (Eq, Show, Read, Generic)

instance ParseField FrontendLanguage
instance ParseRecord FrontendLanguage
instance ParseFields FrontendLanguage

data Tasks
= GenerateRoutes
{ language :: FrontendLanguage "Options are: Typescript, Swift"
, frontendRepo :: FilePath "Path to the root of the frontend repo"
}
deriving (Generic, Show)

instance ParseRecord Tasks
```

It works fine in the success case, though there isn't a specific error message if you provide an invalid sum type, just the generic help text.

Is this the best way to do this? I couldn't find anything in the documentation about this (could have missed it, though). If it is the best way, I'll submit a PR to document this use-case in the Haddocks if you'd like.

Start by reviewing the existing Haddocks for ParseField, ParseRecord, and ParseFields, then check how sum-type parsing and invalid values are currently described. Confirm whether this pattern is supported and how failures are reported; done means documenting the working usage and its error behavior, or recording that the approach needs a separate change.

Written by the indexing model from the issue text.