Add instructions on building as a standalone project

[XLXXLXXLX]

Explain what you would like to see improved

I'm currently studying on the source code of cling. But often cmake detects something and triggers a full compilation, which can waste a lot of time. Also I checked the CMakeLists.txt to see that cling actually supports building as a stand alone project, so I wonder if cling can add some instructions in the document.

Something like this might work.

mkdir cling && cd cling
#llvm configuration
git clone https://github.com/root-project/llvm-project.git
cd llvm-project
git checkout cling-latest
mkdir llvm-build && cd llvm-build
cmake -DLLVM_ENABLE_PROJECTS="clang" -DLLVM_TARGETS_TO_BUILD="host;NVPTX" -DCMAKE_BUILD_TYPE=Release ../llvm
cmake --build . --target all

#cling configuration
cd ..
git clone https://github.com/root-project/cling.git
cd cling && mkdir cling-build && cd cling-build
cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH=../llvm-project/llvm-build  -DLLVM_DIR=../llvm-project/llvm-build/lib/cmake/llvm ..
cmake --build . --target cling