Update Docs on toolchains (was: fix compilation error on Debian)

When compiling on Debian "jessie" using "gcc-arm-none-eabi" toolchain
there is compilation error: toolchain compiler doesn't see system
include directory and throws next error:

/usr/lib/gcc/arm-none-eabi/4.8.2/include/stdint.h:9:26: fatal error:
stdint.h: No such file or directory
 # include_next <stdint.h>

Maybe this patch is just workaround for buggy Debian toolchain, but it works
for me and doesn't break anything.

Signed-off-by: Sam Protsenko <joe.skb7@gmail.com>