[Doc] Git feature

[Ilohama]

Hello,

In the documentation, there's a simplier way to clone the repository :

git clone https://github.com/GamedevFramework/gf.git
cd gf
git submodule update --init
mkdir build
cd build
cmake ../ -DGF_BUILD_GAMES=ON -DGF_BUILD_TOOLS=ON
make

can be change to :

git clone --recursive https://github.com/GamedevFramework/gf.git
cd gf
mkdir build
cd build
cmake ../ -DGF_BUILD_GAMES=ON -DGF_BUILD_TOOLS=ON
make