Add changelog

Author: tchoutri

changelog.d/1604

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+synopsis: Full query string helpers
+prs: #1604
+
+description: {
+  This PR introduces `DeepQuery`, a route combinator that implements a pattern commonly known as deep objects.
+  It builds upon the convention of using `[]` for a list of parameters: 
+  `books?filter[search]=value&filter[author][name]=value`.
+  The corresponding type would be `DeepQuery "filter" BookQuery :> Get '[JSON] [Book]`.
+
+}