Missing step in Installation instruction

- [x] Please add the step to install PyQt5, possibly earlier in the "Install the Plugin" section before "Post Installation":
> Install GUI toolkit PyQt5:
> ```
> pip install -U 'napari[pyqt5]'
> ```

>

- [ ] Also clearly state that at the moment to correctly install our plugin they should follow the instruction and install the plugin with napari, and not installing the plugin from napari UI (or this is actually fine? @yrkim98)

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Here is the feedback from Jianxu's student regarding these two issues:

> Following the instructions, either pyqt5 or pyside2 must be installed manually. If napari is installed first and the plugin is then installed via the napari UI, npe2 must be downgraded to 0.7.6.