Add minimum Flash Memory size for STM32 board requirements

[void269]

In the below Wiki page it only states you need a STM32 based board but does not state how much onboard Flash Memory is needed. Please add the minimum specs and possibly a link or two to known supported boards.
Thanks!

https://github.com/FreeJoy-Team/FreeJoyWiki/blob/master/eng/Flashing-firmware.md

[void269]

Digging deeper it does appear the naming convention does state 64kb of flash; but it would still be nice to have it spelled out for us dodos. ;-)

https://erc-bpgc.github.io/handbook/electronics/Development_Boards/STM32/