Merge pull request #802 from DavidCain/dcain-contributing

Fix `pip install` command for contributors on zsh

CONTRIBUTING.rst

@@ -36,7 +36,7 @@ You want to contribute some code? Great! Here are a few steps to get you started
       $ python -m venv venv
       $ source venv/bin/activate
       (venv) $ python -m pip install -U pip setuptools
-      (venv) $ pip install -U -e .[validation]
+      (venv) $ pip install -U -e '.[validation]'
       (venv) $ pip install -U -r requirements/dev.txt
 
 #. **Make your changes and check them against the test project**