docs(clone): add promise example

lib/constructor.js

@@ -253,6 +253,54 @@ util.inherits(Sharp, stream.Duplex);
  * // firstWritableStream receives auto-rotated, resized readableStream
  * // secondWritableStream receives auto-rotated, extracted region of readableStream
  *
+ * @example
+ * // Create a pipeline that will download an image, resize it and format it to different files
+ * // Using Promises to know when the pipeline is complete
+ * const fs = require("fs");
+ * const got = require("got");
+ * const sharpStream = sharp({
+ *   failOnError: false
+ * });
+ *
+ * const promises = [];
+ *
+ * promises.push(
+ *   sharpStream
+ *     .clone()
+ *     .jpeg({ quality: 100 })
+ *     .toFile("originalFile.jpg")
+ * );
+ *
+ * promises.push(
+ *   sharpStream
+ *     .clone()
+ *     .resize({ width: 500 })
+ *     .jpeg({ quality: 80 })
+ *     .toFile("optimized-500.jpg")
+ * );
+ *
+ * promises.push(
+ *   sharpStream
+ *     .clone()
+ *     .resize({ width: 500 })
+ *     .webp({ quality: 80 })
+ *     .toFile("optimized-500.webp")
+ * );
+ *
+ * // https://github.com/sindresorhus/got#gotstreamurl-options
+ * got.stream("https://www.example.com/some-file.jpg").pipe(sharpStream);
+ *
+ * Promise.all(promises)
+ *   .then(res => { console.log("Done!", res); })
+ *   .catch(err => {
+ *     console.error("Error processing files, let's clean it up", err);
+ *     try {
+ *       fs.unlinkSync("originalFile.jpg");
+ *       fs.unlinkSync("optimized-500.jpg");
+ *       fs.unlinkSync("optimized-500.webp");
+ *     } catch (e) {}
+ *   });
+ *
  * @returns {Sharp}
  */
 function clone () {