Add compile_commands flag to README (openxla#2240)

We don't provide documentation on how to generate compile_commands.json
and this could be a useful start. I left this flag out of CI since the
CI doesn't need to generate this.

README.md

@@ -104,6 +104,7 @@ Here's how to build the StableHLO repo on Linux or macOS:
      -DSTABLEHLO_ENABLE_SPLIT_DWARF=ON \
      -DCMAKE_CXX_COMPILER_LAUNCHER=ccache \
      -DCMAKE_C_COMPILER_LAUNCHER=ccache \
+     -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
      -DSTABLEHLO_ENABLE_SANITIZER=address \
      -DMLIR_DIR=${PWD}/../llvm-build/lib/cmake/mlir