update README with info regarding building toolchain
This repository contains the source for the Phoenix-RTOS microkernel.
IA32, ARM, ARMv7, RISCV64 architectures are supported.
+## Building toolchain
+To build cross-compile toolchain use `toolchain/build-toolchain.sh` script.
+Targets supported currently by the script:
+- i386-pc-phoenix
+- arm-phoenix
+
+Sample invocation:
+```
+cd toolchain
+./build-toolchain.sh arm-phoenix ~/arm-phoenix-toolchain
+```
+
+For other targets You need to build toolchain manually
+
+After successful compilation add the toolchain to the PATH variable, for example:
+```
+export PATH=~/arm-phoenix-toolchain/arm-phoenix/bin/:$PATH
+```
+
+## Building microkernel
To compile microkernel for target architecture edit Makefile and TARGET variable and after this type:
$ make clean
$ make
+## License
This work is licensed under a BSD license. See the LICENSE file for details.
