Update README to fix zarr version requirement

Author: jmuhlich

README.md

@@ -105,7 +105,7 @@ conda activate ashlar
 
 In the activated environment, install dependencies and ashlar itself:
 ```bash
-conda install -y -c conda-forge numpy scipy matplotlib networkx scikit-image scikit-learn tifffile zarr pyjnius blessed
+conda install -y -c conda-forge numpy scipy matplotlib networkx scikit-image scikit-learn tifffile "zarr<3" pyjnius blessed
 pip install ashlar
 ```