Add note about redirecting config for testing.

New contributors might like to be reminded to redirect the config
access of their locally-built qutebrowser to avoid overwriting their
global settings.

INSTALL.asciidoc

@@ -266,6 +266,14 @@ your `$PATH` (e.g. `/usr/local/bin/qutebrowser` or `~/bin/qutebrowser`):
 ~/path/to/qutebrowser/.venv/bin/python3 -m qutebrowser "$@"
 ----
 
+If you are developing on qutebrowser, you may want to redirect it to a local
+config:
+
+----
+#!/bin/bash
+~/path/to/qutebrowser/.venv/bin/python3 -m qutebrowser -c .qutebrowser-local "$@"
+----
+
 Updating
 ~~~~~~~~