Update 0.5 release notes

docs/docs/the-marten-project/release-notes/0.5.md

@@ -148,6 +148,7 @@ Please refer to [Filtering with raw SQL predicates](../../models-and-databases/r
 * The `loop` variable, which is available when using the [`for`](../../templates/reference/tags.md#for) template tag, now supports `even?` and `odd?` attributes.
 * A new [`csrf_input`](../../templates/reference/tags.md#csrf_input) template tag was introduced to make it easier to insert the CSRF token into HTML forms. This new tag allows the generation of a hidden form input containing the CSRF token computed for the current request.
 * A new [`escape`](../../templates/reference/tags.md#escape) template tag was introduced to allow disabling/enabling [auto-escaping](../../templates/introduction.md#auto-escaping) within a block.
+* A new [`unless`](../../templates/reference/tags.md#unless) template tag was introduced to provide an alternative way of writing conditional template blocks.
 
 #### Development