Docs: tile-based output requires libgsf

lib/output.js

@@ -1056,6 +1056,10 @@ function raw (options) {
  *
  * The container will be set to `zip` when the output is a Buffer or Stream, otherwise it will default to `fs`.
  *
+ * Requires libvips compiled with support for libgsf.
+ * The prebuilt binaries do not include this - see
+ * {@link https://sharp.pixelplumbing.com/install#custom-libvips installing a custom libvips}.
+ *
  * @example
  *  sharp('input.tiff')
  *   .png()