Update README.md for conda install change (#6123)

Updates README conda install instructions for the fact that the conda
package no longer pulls Qt—a Qt backend needs to be specified.

# References
conda-forge/napari-feedstock#47
conda-forge/napari-feedstock#48
napari/docs#202

## Type of change
- [x] Documentation

README.md

@@ -31,7 +31,7 @@ conda activate napari-env
 python -m pip install "napari[all]"
 ```
 
-If you prefer conda over pip, you can replace the last line with: `conda install -c conda-forge napari`
+If you prefer conda over pip, you can replace the last line with: `conda install -c conda-forge napari pyqt`
 
 See here for the full [installation guide](https://napari.org/tutorials/fundamentals/installation.html).