fixed installation command from source (#173)

Author: atinfinity

docs/index.md

@@ -35,10 +35,10 @@ What this library does NOT provide:
 sudo apt-get install -y libglm-dev libglfw3-dev libpng-dev libjpeg-dev libeigen3-dev
 
 # Build and install Iridescence
-git clone https://github.com/koide3/iridescence
+git clone https://github.com/koide3/iridescence --recursive
 mkdir iridescence/build && cd iridescence/build
-cmake ..
-make -j
+cmake .. -DCMAKE_BUILD_TYPE=Release
+make -j$(nproc)
 sudo make install
 
 # [Optional] Build and install python bindings