Document upgrading a Python package

It's pretty basic stuff, but I don't otherwise write a lot of Python.

README.md

@@ -115,17 +115,26 @@ Sometimes, elm-package flakes out due to connection issues. The simplest solutio
 with_retry.rb elm-package install
 ```
 
-
 ## elm_self_publish
 
 Sometimes, we want to "install" our packages locally to test them before publishing them remotely. This is designed only with the use case of testing packaged, not using them in production. It doesn't provide any of the guarantees nor support that elm-package does. If you're doing production stuff, elm-package is what you want.
 
-
 ```
 python elm_self_publish.py ../elm-css ../json-to-elm/
 ```
 
 will publish elm-css into json-to-elm
 
+## Upgrading a Python dependency
+
+If you have Nix, you can load a shell with the necessary dependencies like this:
+
+```
+nix-shell -p python37Packages.pip-tools
+```
 
+Once in the shell, run:
 
+```
+pip-compile --upgrade-package my-package
+```