docs(README): link to Why Use Pump?

Add a link to “Why Use Pump?” in the error documentation, so users know
how to capture error messages.

[ci skip]

README.md

@@ -93,14 +93,16 @@ The GulpUglifyError constructor is exported by this plugin for `instanceof` chec
 It contains the following properties:
 
 - `fileName`: The full file path for the file being minified.
-- `cause`: The original UglifyJS error, if avialable.
+- `cause`: The original UglifyJS error, if available.
 
 Most UglifyJS error messages have the following properties:
 
 - `message` (or `msg`)
 - `filename`
 - `line`
 
+To see useful error messages, see [Why Use Pump?](docs/why-use-pump/README.md#why-use-pump).
+
 ## Using a Different UglifyJS
 
 By default, `gulp-uglify` uses the version of UglifyJS installed as a dependency.