add developer documentation to ppusim/ppu interface

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+# src (game source files)
+
+- the game can be built for the stm32 with fpga, or for desktop (windows and
+  linux tested), by editing the `PLATFORM` variable in `makefile`
+- stm-specific sources should be in the stm32 folder (setup/configuration code
+  using HAL or FreeRTOS), or in `stm32.c` in any subfolder (when used as
+  interface implementation)
+- stm-specific sources should be declared in `stm32.mk`
+- desktop-specific sources should be declared in `ds.mk`
+- other source files (both platforms) should be declared in `makefile`
+- for small sections of platform specific code, `#ifdef` macro's can be used.
+  either `HH_TARGET_STM32` or `HH_TARGET_DESKTOP` is defined.
+
+## directory structure
+
+|name|description|
+|apu|APU interface functions|
+|lib|libraries (stm system headers, HAL, FreeRTOS)|
+|ppusim|PPU simulation code (desktop only)|
+|ppu|PPU interface functions|
+|static|static game assets (binary files)|
+|stm32|STM32 specific setup (HAL, FreeRTOS)|