Add docs for custom ranges

docs/components/forms.md

@@ -928,3 +928,11 @@ $custom-file-text: (
 {% endhighlight %}
 
 You'll need to set the language of your document (or subtree thereof) correctly in order for the correct text to be shown. This can be done using [the `lang` attribute](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/lang) or the [`Content-Language` HTTP header](https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.12), among other methods.
+
+### Range input
+
+Custom `<input type="range">` need only a custom class, `.custom-range` to trigger the custom styles.
+
+{% example html %}
+<input type="range" class="custom-range">
+{% endexample %}