Doc for image property

[leorouxx]

hello, I have noticed that the README contains an empty field when it comes to describe the "image" property. I don't know if this is relevant, but I have done the following :

image properties

Property	Description
page_number	Page number on which the image was found.
height	Height of the image.
width	Width of the image.
x0	Distance of left side of the image from left side of page.
x1	Distance of right side of the image from left side of page.
y0	Distance of bottom of the image from bottom of page.
y1	Distance of top of the image from bottom of page.
top	Distance of top of the image from top of page.
bottom	Distance of bottom of the image from top of page.
doctop	Distance of top of rectangle from top of document.
mcid	The marked content section ID for this image if any (otherwise None). Experimental attribute.
tag	The marked content section tag for this image if any (otherwise None). Experimental attribute.
srcsize	Original Dimension of the image, in pixel (tuple (width, height)).
linewidth	Thickness of line.
stream	Pixel values of the image.
imagemask	#TODO
bits	Encoding of the image (8 for 0-255 range)
colorspace	Color domain of the image (e.g. RGB)
object_type	"image"

As you can see, I lack the 'imagemask' value, because I could find PDF where this value is not None, and from this documentation, it seems that it would be a stream-like object. I'm not sure so if any of you actually know what it is I would love to know as well.
Also, please consider giving your feedback on fields you consider not well filled.

Thanks.

[jsvine]

Many thanks for this suggestion, @leorouxx. I've added that documentation — with just a few edits of your proposal, plus credit to you — here: 937702d