Add 'append' documentation

_filters/append.md

@@ -0,0 +1,22 @@
+---
+title: append
+---
+
+`append` concatenates two strings and returns the concatenated value.
+
+{% highlight liquid %}
+{% raw %}
+{{ "/my/fancy/url" | append:".html" }}
+{% endraw %}
+# => "/my/fancy/url.html"
+{% endhighlight %}
+
+It can also be used with variables:
+
+{% highlight liquid %}
+{% raw %}
+{% assign filename = "/index.html" %}
+{{ product.url | append:filename }}
+{% endraw %}
+# => "#{product.url}/index.html"
+{% endhighlight %}