We depend on pytz, but until late last year we got it implicitly through
depending on Django. Since their release 4.0, however, they no longer
depend on pytz; commit 250479dc3 added the dependency directly to our
metadata in setup.py, but the documentation about dependencies (most
importantly, the instructions for new contributors) was left untouched.
This commit updates the new contributor instructions to suggest an
"editable installation" of the project at the step that previously had
users manually install Django. In this mode, pip fetches and installs
the project dependencies automatically (so in the unlikely event we grow
another dependency, that doc doesn't need to be changed again) and makes
the project available to the virtualenv's python as a normal package,
but doesn't require reinstallation for mundane edits.
