Fix usage instructions in README (#113)

Refs #79

This change updates the usage instructions in the README to explain
the workaround required to use this module. As userland modules do not
hide core modules, a trailing slash is required to have Node.js use this
module over the built-in.

README.md

@@ -24,8 +24,12 @@ npm install punycode --save
 
 In [Node.js](https://nodejs.org/):
 
+> ⚠️ Note that userland modules don't hide core modules.
+> For example, `require('punycode')` still imports the deprecated core module even if you executed `npm install punycode`.
+> Use `require('punycode/')` to import userland modules rather than core modules.
+
 ```js
-const punycode = require('punycode');
+const punycode = require('punycode/');
 ```
 
 ## API