Update README for recursive clone

README.md

@@ -9,7 +9,7 @@ This repository contains the three subprojects of Image Domain Gridding:
 
 # Installation
 ```
-git clone https://git.astron.nl/RD/idg.git
+git clone https://git.astron.nl/RD/idg.git --recursive
 cd idg && mkdir build && cd build
 cmake -DCMAKE_INSTALL_PREFIX=/usr/local/idg/ ..
 ccmake . # For interactively setting parameters, like BUILD_LIB_CUDA