Add instructions for Windows install to docs

[ras44]

Steps to conda/pip install in windows:

• install anaconda for windows

• open anaconda powershell prompt (NOT the regular anaconda prompt nor a jupyter notebook shell prompt) and follow the instructions to create and activate a clean conda environment

• Download and install the C++ Build tools from https://visualstudio.microsoft.com/visual-cpp-build-tools/
  

• Once the installation is complete, click on "Modify":
  

• Then click on and install the "Desktop development with C++" workload. This will install about 300 packages, currently 6.1 GB.

• Once installation is complete, open up a Developer Command Prompt (windows button + type "developer command prompt") and execute the command 'cl'. This should produce no errors and produce output similar to:

• now go back to your anaconda powershell prompt and run

git clone https://github.com/uber/causalml.git
cd causalml
pip install .
python setup.py build_ext --inplace

• This should install the latest causalml code into your conda environment.

Troubleshooting

See additional helpful comments in #665 (comment) for installing with miniconda.