Document dependency on imagemagick

[jldoubleu]

I attempted to build 8bkc-fluppy following the setup instructions in the Getting Started section of the documentation. The build failed with:

make[1]: convert: Command not found
/esp/8bkc-sdk/8bkc-components/powerbtn_menu/component.mk:16: recipe for target 'graphics.inc' failed
make[1]: *** [graphics.inc] Error 127
/esp/8bkc-sdk/esp-idf/make/project.mk:450: recipe for target 'component-powerbtn_menu-build' failed
make: *** [component-powerbtn_menu-build] Error 2

installing imagemagick resolved the issue

[Spritetm]

I merged your PR, thanks!