function joinChannel (images, options) {
  if (Array.isArray(images)) {
    images.forEach(function (image) {
      this.options.joinChannelIn.push(this._createInputDescriptor(image, options));
    }, this);
  } else {
    this.options.joinChannelIn.push(this._createInputDescriptor(images, options));
  }
  return this;
}
Join one or more channels to the image.
The meaning of the added channels depends on the output colourspace, set with `toColourspace()`.
By default the output image will be web-friendly sRGB, with additional channels interpreted as alpha channels.
Channel ordering follows vips convention:
- sRGB: 0: Red, 1: Green, 2: Blue, 3: Alpha.
- CMYK: 0: Magenta, 1: Cyan, 2: Yellow, 3: Black, 4: Alpha.

Buffers may be any of the image formats supported by sharp.
For raw pixel input, the `options` object should contain a `raw` attribute, which follows the format of the attribute of the same name in the `sharp()` constructor.

@param {Array<string|Buffer>|string|Buffer} images - one or more images (file paths, Buffers).
@param {Object} options - image options, see `sharp()` constructor.
@returns {Sharp}
@throws {Error} Invalid parameters