Readme refactoring (#2038)

wamr-compiler/README.md

@@ -0,0 +1,23 @@
+
+### Build wamrc AOT compiler
+
+Both wasm binary file and AOT file are supported by iwasm. The wamrc AOT compiler is to compile wasm binary file to AOT file which can also be run by iwasm. Execute following commands to build **wamrc** compiler for Linux:
+
+```shell
+cd wamr-compiler
+./build_llvm.sh (or "./build_llvm_xtensa.sh" to support xtensa target)
+mkdir build && cd build
+cmake .. (or "cmake .. -DWAMR_BUILD_PLATFORM=darwin" for MacOS)
+make
+# wamrc is generated under current directory
+```
+
+For **Windows**：
+```shell
+cd wamr-compiler
+python build_llvm.py
+mkdir build && cd build
+cmake ..
+cmake --build . --config Release
+# wamrc.exe is generated under .\Release directory
+```