[Binding] update README about LLVM build with python binding (#299)

Author: Yun-Fly

README.md

@@ -35,6 +35,8 @@ Notes
  * It is recommended to add optional options `-DLLVM_BUILD_LLVM_DYLIB=ON -DLLVM_LINK_LLVM_DYLIB=ON` to the command `cmake -G Ninja llvm ...` above **if you are building for CPU only**. These will enable the build of LLVM/MLIR dynamic libraries and let MLIR/LLVM tools link to them, to reduce the installed binary size of LLVM/MLIR. These options also enable the `GC_DEV_LINK_LLVM_DYLIB` option of graph-compiler repo (see below).
  * The option `-DLLVM_INSTALL_GTEST=ON` is optional, if the tests of graph-compiler are disabled (see `GC_ENABLE_TEST` below).
  * If you would like to enable GPU components of Graph Compiler, please make sure to statically link Graph Compiler and LLVM(MLIR). It is a known issue that LLVM shared library cannot be linked together with IGC (Intel's low level GPU compiler). Make sure `LLVM_BUILD_LLVM_DYLIB` and `LLVM_LINK_LLVM_DYLIB` are `OFF` (they are off by default). Also make sure Graph Compiler's cmake option `GC_DEV_LINK_LLVM_DYLIB` is `OFF` when configuring Graph Compiler (see below).
+* If you would like to enable [`BenchGC`](docs/benchgc_overview.md) or [`MLIR Binding`](python/README.md), please confirm to add options
+ ``-DPython3_EXECUTABLE=`which python3` -DMLIR_ENABLE_BINDINGS_PYTHON=ON``.
 
 We have now installed LLVM at `llvm-project/llvm-install`.