README: add --recusive to git clone to make -DZXING_WRITER=NEW …

…work out of the box

README.md

@@ -104,7 +104,7 @@ These are the generic instructions to build the library on Windows/macOS/Linux.
 3. See the cmake `ZXING_...` options to enable the testing code, python wrapper, etc.
 
 ```
-git clone https://github.com/zxing-cpp/zxing-cpp.git --single-branch --depth 1
+git clone https://github.com/zxing-cpp/zxing-cpp.git --recursive --single-branch --depth 1
 cmake -S zxing-cpp -B zxing-cpp.release -DCMAKE_BUILD_TYPE=Release
 cmake --build zxing-cpp.release -j8 --config Release
 ```