Document use of PDB in dev env

[heartsucker]

Description

Reporting this here while documenting TODOs (#724).

From securedrop/management/run.py:

    def monitor(self):
        while True:
            # TODO: we currently don't handle input, which makes using an
            # interactive debugger like pdb impossible. Since Flask provides
            # a featureful in-browser debugger, I'll accept that pdb is
            # broken for now. If someone really wants it, they should be
            # able to change this function to make it work (although I'm not
            # sure how hard that would be).
            #
            # If you really want to use pdb, you can just run the
            # application scripts individually (`python source.py` or
            # `python journalist.py`).
            rprocs, _, _ = select.select(self.procs, [], [])

[redshiftzero]

I am pdb's biggest fan, and even I don't find this annoying. What we should do however is document how to run journalist.py or source.py using bin/dev-shell so one can easily put in breakpoints