Add example to filtering page (#295)

Co-authored-by: Benjie Gillam <benjie@jemjie.com>

Author: BenjieGillam

src/pages/postgraphile/filtering.md

@@ -9,6 +9,8 @@ Out of the box, PostGraphile supports rudimentary filtering on
 allows you to filter for specific values (e.g. `username: "Alice"` or
 `category: ARTICLE`).
 
+[See an example using the `connection` argument.](/postgraphile/examples/#Collections__Relation_condition)
+
 It's important when implementing filters to keep performance in mind, so
 PostGraphile gives you the ability to omit certain fields from the list of
 filters using the `@omit filter` [smart comment](/postgraphile/smart-comments/).