add stm32f40x/readme.txt

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2240 bbd45198-f89e-11dd-88c7-29a3b14d5316

bsp/stm32f40x/readme.txt

+board info:
+STM32F4 discovery
+http://www.st.com/internet/evalboard/product/252419.jsp
+
+note:
+in drivers/stm32f4xx_conf.h
+ /* Redefine the HSE value; it's equal to 8 MHz on the STM32F4-DISCOVERY Kit */
+ #define HSE_VALUE    ((uint32_t)8000000)
+