adding initial README.md

This patch adds initial README.md on usage.

Signed-off-by: Yanfeng Liu <[email protected]>

README.md

@@ -0,0 +1,34 @@
+# Introduction
+
+This builds Linux with rv64ilp32 for trying on K230 devices.
+
+# Preparations
+
+Please prepare toolchain and source tree before building.
+
+## Toolchain preparation
+
+Please get the [rv64ilp32 toolchain](https://github.com/ruyisdk/riscv-gnu-toolchain-rv64ilp32) and install on build host.
+
+Please setup environment variable `RV64ILP32_TOOLCHAIN_HOME` as the toolchain installation path:
+
+```
+$ export RV64ILP32_TOOLCHAIN_HOME=/opt/rv64ilp32
+$ ls -F $RV64ILP32_TOOLCHAIN_HOME
+riscv/
+```
+## Source tree preparation
+
+Before building, please run the following commands to fetch the submodule sources:
+
+```
+$ git submodule update --init --recursive
+```
+
+# Build
+
+Run `build.sh` in top folder to see how to build different targets.
+
+The final image will be available in `output/` folder, which can be used on K230devices.
+
+