Explain how to override default parcels versoin

[willirath]

Based on the default workflow:

$ git clone https://github.com/Parcels-code/parcels-benchmarks.git
$ cd parcels-benchmarks
$ pixi install
$ pixi run python run_benchmarks  # currently uses Parcels from the conda alpha package

Overriding based on Git ref (e.g. v4-dev) works as:

$ pixi add --pypi "parcels @ git+https://github.com/Parcels-code/parcels.git@v4-dev"
$ pixi run python run_benchmarks  # then runs in HEAD of the v4-dev branch