README: add caution of toolchain usages for 64bit OS
Our recommanded toolchain is fully working on 32bits OS.
64bits OS needs to install additional ubuntu package.
This commit describes above.
@@ -22,6 +22,7 @@ Untar the gcc-arm-none-eabi-4_9-2015q3-20150921-linux.tar.bz2 and export the pat
tar xvjf gcc-arm-none-eabi-4_9-2015q3-20150921-linux.tar.bz2
export PATH=<Your Toolchain PATH>:$PATH
```
+Be aware that recommanded toolchain is fully working on 32bits machine. 64bits machine needs additional packages.
### Getting the sources
