Document sbt-digest's `indexWriter` to make use of fingerprinted assets in JavaScript

- https://github.com/orgs/playframework/discussions/13514
- https://github.com/playframework/playframework/issues/3435#issuecomment-57749755
- https://github.com/mkurz/sbt-digest-indexwriter

Specially useful for client side frameworks like React and Svelte.