Document what to do when pipenv kills everything

Pipenv recently murdered the entire workflow. This turned out to be key:
```
All dependencies are now up-to-date!
Upgrading  in  dependencies.
```

`pipenv update --outdated`  is necessary to show what pipenv decided arbitrarily to mess with, then `pipenv upgrade packagename` was necessary to fix, git add Pipfile Pipfile.lock, commit, push.