Instructions:
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1.) Put your to-be-classified images, named 'image_1.png', 'image_2.png', ...
    in the subdirectory 'classifyimages/'.

2.) Start a local PHP server to run the script:
    $ php -S localhost:7777

3.) Open your favourite web browser and go to the site
    http://localhost:7777/classify.php

4.) There should now be a dialogue box asking you to enter a file name.
    Any name is fine, this will be the name of an ASCII file holding your
    classification results. If you come back after partially completing
    the classification, enter the same name as before to resume classifying.

5.) There should then appear a series of galaxy images, one by one, with
    the option to tick a few simple boxes beneath. Clicking 'Next galaxy'
    will switch to the next one; for completeness, your answer will be 
    shown near the top of the screen (in numerical values).

6.) 'No' gives a score of 1, 'Maybe' 2, and 'Yes' 3. For 'Special?', ticking
    the box gives a value of 1 (else 0). Anything entered in the 'Comment'
    field will be recorded as is.

7.) When all galaxy images have been shown, the program will display a short
    message, you can then quit the browser. Otherwise, you can take a break
    at any point by just quitting the browser, and then resume by repeating
    steps 2--4 and entering the same file name in step 4.

8.) The file ./doc/[whatever name you entered] contains an ASCII list of your
    results. You can do with it whatever you want...