Added depth, flatten & colorspace to README

Author: Andrei Tomita

README.md

@@ -164,6 +164,9 @@ This is why a "few" ones are implemented now, to make sure validation is possibl
 * [`-thumbnail`](http://www.imagemagick.org/script/command-line-options.php#thumbnail)
 * [`-annotate`](http://www.imagemagick.org/script/command-line-options.php#annotate)
 * [`-draw`](http://www.imagemagick.org/script/command-line-options.php#draw)
+* [`-depth`](http://www.imagemagick.org/script/command-line-options.php#depth)
+* [`-flatten`](http://www.imagemagick.org/script/command-line-options.php#flatten)
+* [`-colorspace`](http://www.imagemagick.org/script/command-line-options.php#colorspace)
 * [`xc:`](http://www.imagemagick.org/Usage/canvas/)
 
 Feel free to ask/create an issue if you need more!