03-packaging-installing: Add tip about `pip install -e`

The material in episode 2 could be improved by mentioning (maybe in a callout box) `pip install -e`, which allows the latest version of a module/package under active development to be imported and used without the need to re-install after every set of changes. (See line 351 of [the etherpad from CC@Home2020](https://pad.carpentries.org/cchome-python-package-publish).)

Locate the episode 2 material for 03-packaging-installing and review line 351 of the linked CC@Home2020 etherpad for context. Add a tip or callout explaining `pip install -e` and confirm that the lesson presents the command and its active-development use case clearly.

Written by the indexing model from the issue text.