Recommend the -U switch when running pip install -e .

This more reliably ensures that the user has up-to-date dependencies (e.g. without it, a
beta version of a package like Willow or modelcluster won't get installed if the user has
a stable version already installed)

docs/contributing/developing.rst

@@ -26,7 +26,7 @@ With your preferred virtualenv activated, install the Wagtail package in develop
 
 .. code-block:: sh
 
-    pip install -e .[testing,docs]
+    pip install -e .[testing,docs] -U
 
 Install the tool chain for building static assets: