Instructions how to contribute to documentation

[jbusecke]

I was just trying to make some changes to the docs tutorial for #93, and found it quite hard to get the right dependencies.

This is what I had to do to

conda create --name rechunker_docs python
conda activate rechunker_docs
# this step took quite long for me to execute
pip install -e .[docs] 
# pip install -e .\[docs\] #for zsh shell
# additional install needed for the notebook to run properly
conda install ipykernel pooch netcdf4 zarr dask distributed

I am sure this is not the most elegant way to set this up. I think it would really help other folks to have a little guide on this. I am happy to write that up, if someone could check and see if there isnt a better way to set this up.