Update README MCU/boards (#17827)

README.md

@@ -69,6 +69,12 @@ Marlin 2.0 introduces a layer of abstraction so that all the existing high-level
   boards|processor|speed|flash|sram|logic|fpu
   ----|---------|-----|-----|----|-----|---
   [Arduino STM32](https://github.com/rogerclarkmelbourne/Arduino_STM32)|[STM32F1](https://www.st.com/en/microcontrollers-microprocessors/stm32f103.html) ARM-Cortex M3|72MHz|256-512k|48-64k|3.3V|no
+  [Geeetech3D GTM32](https://github.com/Geeetech3D/Diagram/blob/master/Rostock301/Hardware_GTM32_PRO_VB.pdf)|[STM32F1](https://www.st.com/en/microcontrollers-microprocessors/stm32f103.html) ARM-Cortex M3|72MHz|256-512k|48-64k|3.3V|no
+ 
+  #### STM32F4
+
+  boards|processor|speed|flash|sram|logic|fpu
+  ----|---------|-----|-----|----|-----|---
   [STEVAL-3DP001V1](http://www.st.com/en/evaluation-tools/steval-3dp001v1.html)|[STM32F401VE Arm-Cortex M4](http://www.st.com/en/microcontrollers/stm32f401ve.html)|84MHz|512k|64+32k|3.3-5V|yes
 
   #### Teensy++ 2.0