[target][stm32f4-discovery] add readme for this target
[target][stm32f4-discovery] add readme for this target

- pointers to manufacture website and digikey
- details on where the debug/console uart is
- details on how to connect an external swd debugger

sndnvaps/lk,ErikCorryGoogle/lk,travisg/lk,littlekernel/lk,ErikCorryGoogle/lk,travisg/lk,sndnvaps/lk,hollanderic/lkstuff,sndnvaps/lk,hollanderic/lkstuff,minglun-tsai/lk,minglun-tsai/lk,minglun-tsai/lk,sndnvaps/lk,travisg/lk,ErikCorryGoogle/lk,minglun-tsai/lk,littlekernel/lk,hollanderic/lkstuff,littlekernel/lk,sndnvaps/lk,hollanderic/lkstuff,littlekernel/lk,travisg/lk,ErikCorryGoogle/lk