Add docs for default

_filters/default.md

@@ -0,0 +1,22 @@
+---
+title: default
+---
+
+`default` offers a means of having a fall-back value in case your value doesn't exist.
+
+{% highlight liquid %}
+{% raw %}
+{{ product_price | default:2.99 }}
+// => outputs "2.99"
+
+{% assign product_price = 4.99 %}
+{{ product_price | default:2.99 }}
+// => outputs "4.99"
+
+{% assign product_price = "" %}
+{{ product_price | default:2.99 }}
+// => outputs "2.99"
+{% endraw %}
+{% endhighlight %}
+
+`default` will use its substitute if the left side is `nil`, `false`, or empty.