[Maint] Improve contribution in README and remove pyqt5 from [dev] install dependency

[psobolewskiPhD]

Closes: #185
Closes: #168

I've removed pyqt5 from [dev] and added a Note in the Contributing section that you need a suitable Qt backend for napari. I've added an example using a conda virtual env and installing [dev] and PyQt5. Finally, I explicitly provide the command to run tests locally.

[jni]

@psobolewskiPhD should we link to https://napari.org/dev/tutorials/fundamentals/installation.html?highlight=qt%20backend#choosing-a-different-qt-backend?

I'm using the dev link because backends seem to be something where updates can happen independently of the napari version.

[psobolewskiPhD]

Sure I can add that. The gist is you need a functioning napari install, which means having a Qt backend. But if you have napari with pyside or pyqt6 or whatever this plugin shouldn't override that with pyqt5.

[psobolewskiPhD]

I updated the PR (and OP) to add the requested link and to address #168 also.
The shell block followed by pytest . works for me locally--all green!--with this PR.