docs: add imagemagick prereq for linux

[botamochi0x12]

Abstract

This PR adds the line about the prerequisite imagemagick for Linux.

Background

The PR #108 fixed by including imagemagick and another however imagemagick was not added to README.md.

https://github.com/westonplatter/phashion/pull/108/changes#diff-b803fcb7f17ed9235f1e5cb1fcd2f5d3b2838429d4368ae4c57ce4436577f03fR20

phashion/.github/workflows/ci.yml

Line 20 in 0eb5677

run: sudo apt-get install -y libpng-dev libjpeg-dev libsqlite3-dev imagemagick

https://github.com/westonplatter/phashion/pull/108/changes#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5R38-R41

phashion/README.md

Lines 35 to 41 in 0eb5677

	Linux
	- libjpeg-dev
	- libpng-dev
	- libsqlite3-dev
	Mac
	- [imagemagick](http://www.imagemagick.org/)

Running the following code on a codespace fails::

x.x.x :xxx > require "phashion"
3.4.7 :006 > broccoli = Phashion::Image.new("test/gif/Broccoli_Super_Food.gif")
 => #<Phashion::Image:0x00007a3b808c2ef0 @filename="test/gif/Broccoli_Super_Food.gif"> 
3.4.7 :007 > broccoli.duplicate?(broccoli)
sh: 1: convert: not found
sh: 1: gm: not found
[[...Trimmed...]]

References

• add libsqlite3-dev and imagemagick deps, fix #44 #107 #108