Add a react version of the demo notebook. (#4)

.gitignore

@@ -19,3 +19,6 @@ docs/_autosummary
 
 # Notebook checkpoints.
 .ipynb_checkpoints
+
+# Node modules.
+node_modules

react/README.md

@@ -0,0 +1,13 @@
+# REM React Demo
+
+This demonstration illustrates how to use [react](https://react.dev/) to call 
+Rewiring America's Residential Electrification(REM) API. It constructs a web page
+that allows users to type in their address and current heating fuel and get an
+estimate of how much they could save by switching to a heat pump.
+
+To the user, the final result looks a lot like our [HTML/JavaScript demo](../www).
+The only difference is that this version is written in react.
+
+If you are not already familiar with react, we suggest you have a look
+at the [Tic-Tac-Toe Tutorial](https://react.dev/learn/tutorial-tic-tac-toe),
+which is a great interactive introduction to react.