Update CONTRIBUTING.md to include [test] and mention Qt backend (#276)

* Update CONTRIBUTING.md to include [test] and mention Qt backend

* add superqt[test,pyqt6] to dev, mention it in contributing guide

CONTRIBUTING.md

@@ -12,7 +12,7 @@ To get started fork this repository, and clone your fork:
 git clone https://github.com/<your_organization>/superqt
 cd superqt
 
-# install in editable mode
+# install in editable mode (this will install PyQt6 as the Qt backend)
 pip install -e .[dev]
 
 # install pre-commit hooks

pyproject.toml

@@ -65,6 +65,7 @@ dev = [
     "pydocstyle",
     "rich",
     "types-Pygments",
+    "superqt[test,pyqt6]",
 ]
 docs = [
     "mkdocs-macros-plugin ==1.3.7",