Add warning about possible concurrent tile race #1151

lib/output.js

@@ -390,6 +390,8 @@ function toFormat (format, options) {
  * Set the format and options for tile images via the `toFormat`, `jpeg`, `png` or `webp` functions.
  * Use a `.zip` or `.szi` file extension with `toFile` to write to a compressed archive file format.
  *
+ * Warning: multiple sharp instances concurrently producing tile output can expose a possible race condition in some versions of libgsf.
+ *
  * @example
  *  sharp('input.tiff')
  *   .png()