[demo][stm32f405rg] add readme.

demos/stm32f405rg/README.md

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+# stm32f405rg on-chip flash demo
+
+## What
+
+KVDB  and TSDB demo on STM32F4X chip
+
+- MCU: STM32F405RG
+- Flash Driver
+  - STM32 on-chip flash
+- IO
+  - UART
+    - TXD: PA9
+    - RXD: PA10
+
+## How
+
+### Step1: connect the serial terminal
+
+Connect the board to PC by serial port and open the PC serial terminal.
+
+### Step2: open demo project
+
+support 2 kinds of IDEs
+
+- Keil MDK: open `RVMDK\FlashDB.uvprojx`
+- [RT-Thread Studio](https://www.rt-thread.io/studio.html): import this folder by import wizard
+
+### Step3: build and download
+
+Download the firmware to your board when build successful.
+
+### Step4: check the log
+
+This demo's log will output to PC serial.
+