update README

Author: mbostock

README.md

@@ -12,7 +12,7 @@
 
 In Observable notebooks, Plot and D3 are available by default as part of the [standard library](https://observablehq.com/@observablehq/recommended-libraries).
 
-For use with Webpack, Rollup, or other Node-based bundlers, Plot is typically installed via a package manager such as Yarn or npm.
+For use with Webpack, Rollup, or other Node-based bundlers, Plot is typically installed via a package manager such as Yarn or npm. (Plot is distributed as an ES module; see [Sindre Sorhus’s guide](https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c) for help upgrading.)
 
 ```bash
 yarn add @observablehq/plot