write section about partially manual image command creation

[ghost]

a) run buildglyphimages, then change gen_src

b) create the pdf independently, using LilyPond -dpreview or any other (graphics) program

Then use genGlyphCommands.py to create the command

[uliska]

No:

• put manually created .ly file in glyphimages/custom_src
• run script on this file

Or:

• create an image file in any convenient way and put it in glyphimages/custom_images
• run lily-glyph-commands.py on this