The documentation for "adding React to existing app" was updated this week by Dan Abramov to make the installation of React into any website a much simpler process.
Dan writes in the opening paragraphs of the updated docs.
React has been designed from the start for gradual adoption. You can use as little or as much React as you need. Perhaps you only want to add some “sprinkles of interactivity” to an existing page. React components are a great way to do that.
The majority of websites aren’t, and don’t need to be, single-page apps. With a few lines of code and no build tooling, try React in a small part of your website. You can then either gradually expand its presence, or keep it contained to a few dynamic widgets.
Here's the GitHub issue that kicked things off which might provide a slight bit more context.