Update docs for ESM

Author: wooorm

readme.md

@@ -14,6 +14,9 @@ Markdown component for React using [**remark**][remark].
 
 ## Install
 
+This package is [ESM only](https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c):
+Node 12+ is needed to use it and it must be `import`ed instead of `require`d.
+
 [npm][]:
 
 ```sh
@@ -84,6 +87,9 @@ ReactDom.render(
 
 ## API
 
+This package exports the following identifier: `uriTransformer`.
+The default export is `ReactMarkdown`.
+
 ### `props`
 
 *   `children` (`string`, default: `''`)\