docs: add instructions for browser usage to parse current page (#231)

README.md

@@ -23,6 +23,10 @@ yarn add @postlight/mercury-parser
 import Mercury from '@postlight/mercury-parser';
 
 Mercury.parse(url).then(result => console.log(result););
+
+// NOTE: When used in the browser, you can omit the URL argument
+// and simply run `Mercury.parse()` to parse the current page.
+
 ```
 
 The result looks like this: