Add document of how to use this example.

README.md

@@ -1,4 +1,53 @@
-STM32F4-FreeRTOS
-================
+# STM32F4-FreeRTOS
 
 A demo project of FreeRTOS running on a STM32F4 Discovery board.
+
+## Steps to run this example
+
+### Prerequisite
+
+1. A PC running Linux or Windows with Cygwin(not tested). A Mac is also fine with this example.
+2. A STM32F4Discovery board.
+3. A FT232RL USB to serial board which is recommended if there's no serial port on your computer.
+4. USB Cable, Dupont Line and other tools.
+
+### Install the toolchain
+
+The pre-built version of GNU Tools for ARM can be downloaded from its [website](https://launchpad.net/gcc-arm-embedded). It's available for most systems. Follow the instructions in the readme file and installed the toolchain to your system. To verify your installation, simply type `arm-none-eabi-gcc --version` in your terminal, if everything goes right, you'll get output like this:
+
+```
+arm-none-eabi-gcc (GNU Tools for ARM Embedded Processors) 4.7.3 20130312 (release) [ARM/embedded-4_7-branch revision 196615]
+Copyright (C) 2012 Free Software Foundation, Inc.
+This is free software; see the source for copying conditions.  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+```
+
+### Install ST-Link utility
+
+#### Windows
+Grab the official utility from [ST website](http://www.st.com/web/catalog/tools/FM146/CL1984/SC724/SS1677/PF251168). Note that you should install the USB driver before install the st-util.
+
+#### Linux and OS X
+Clone this [git](https://github.com/texane/stlink), follow the instructions on that page and install st-util to your system.
+
+### Compile this example
+The only thing you need to do is to edit the makefile and let it know your toolchain installation path. Change the `TOOLCHARN_ROOT` variable at the third line of makefile and point it to where you installed the toolchain. The you can simply type `make` and compile the example.
+
+### Debug
+Connect your STM32F4Discovery with a USB cable. You can flash the binary into the board with this:
+
+`$ st-flash write binary/FreeRTOS.bin 0x8000000`
+
+The code is wrote directly into internal flash of STM32 processor and it starts to run after reset. To debug it, first start the GDB server:
+
+`$ st-util &`
+
+And then GDB:
+
+```
+$ arm-none-eabi-gdb binary/FreeRTOS.elf
+(gdb) tar ext :4242
+(gdb) b main
+(gdb) c
+```
+
+You'll get breakpoint triggered at `main` function, and enjoy!