Update installation instructions

[PatBall1]

This seems a little out of date: https://patball1.github.io/detectree2/installation.html

Recently got the package working on our cluster with this sequence:

conda create -n dtree2 python==3.10
conda activate dtree2
conda install gdal pygeos geopandas numpy opencv -c conda-forge
pip3 install torch torchvision torchaudio --index-url https://download.pytorch.org/whl/cu118
pip install git+https://github.com/PatBall1/detectree2.git

Should do some more testing to see what is the most flexible/robust approach.