doc: explain es5+esm

Author: gfx

README.md

@@ -521,11 +521,11 @@ Note that `Buffer.from()` for `JSON.stringify()` is necessary to emulate I/O whe
 
 The NPM package distributed in npmjs.com includes both ES2015+ and ES5 files:
 
-* `dist/` is compiled into ES2015+
-* `dist.es5+umd/` is compiled into ES5 and bundled to singile file
+* `dist/` is compiled into ES2015+, provided for NodeJS v10 or later
+* `dist.es5+umd/` is compiled into ES5 with UMD-style single file
   * `dist.es5+umd/msgpack.min.js` - the default, minified file (UMD)
   * `dist.es5+umd/msgpack.js` - an optional, non-minified file (UMD)
-* `dist.es5+esm/` is compiled into ES5 and placed as ES modules
+* `dist.es5+esm/` is compiled into ES5 and placed as ES modules, provided for webpack-like bundlers, not NodeJS
 
 If you use NodeJS and/or webpack, their module resolvers use the suitable one automatically.