Update README and test documentation for clang version consistency

Author: yuqisun

README.md

@@ -7,6 +7,10 @@ Build LLVM & Neura
 $ git clone --depth 1 --branch release/19.x https://github.com/llvm/llvm-project.git
 ```
  - Build LLVM:
+ > **Note:**  
+> `clang` is optional for building Neura itself, but installing and building `clang` together with `mlir` (by setting `-DLLVM_ENABLE_PROJECTS="clang;mlir"`) ensures that you have a `clang`/`clang++` matching the same LLVM/MLIR version (19).  
+> This is recommended if you want to use `clang++` for C++ to LLVM IR/MLIR workflows and to avoid version mismatch issues.
+
 ```sh
  $ cd llvm-project
  $ mkdir build && cd build

test/c2llvm2mlir/test.mlir

@@ -2,7 +2,7 @@
 // We recommend using the clang++ built from the same LLVM/MLIR 19.x source.
 // Please add your build directory to PATH before running tests:
 // `export PATH=/path/to/llvm-project/build/bin:$PATH`
-// This ensures all clang++ invocations use the correct version.
+// This ensures all clang++ invocations use the same version.
 
 // Compiles the original kernel.
 // RUN: clang++ kernel.cpp -o %t-kernel.out