update docs

examples/readme.md

@@ -1,3 +1,38 @@
 # Examples
 
-Build and Run example instructions
+## Clone this repo
+
+```
+$ git clone https://github.com/hathach/tinyusb tinyusb
+$ cd tinyusb
+```
+
+## Fetch submodule MCUs drivers
+
+TinyUSB examples includes external repos aka submodules to provide low-level MCU peripheral's driver to compile with. Therefore we will firstly fetch those mcu driver repo by running this command in the top folder repo
+
+```
+$ git submodule update --init --rescursive
+```
+
+It will takes a bit of time due to the number of supported MCUs, luckily we only need to do this once.
+
+## Build
+
+[Here is the list of supported Boards](docs/boards.md) that should work out of the box with provided examples. 
+ 
+To build example, go to its folder project then type `make BOARD=[our_board] all` e.g
+
+```
+$ cd examples/device/cdc_msc_hid
+$ make BOARD=feather_nrf52840_express all
+```
+
+## Flash
+
+TODO: write more on flashing
+
+```
+$ make BOARD=feather_nrf52840_express flash
+```
+