add: add "Vary" header to responses

Co-authored-by: Steve Chavez <stevechavezast@gmail.com>

docs/references/api/vary_header.rst

@@ -0,0 +1,16 @@
+.. _vary_header:
+
+Vary Header
+===========
+
+In order to assist caching proxies and CDNs, PostgREST includes a ``Vary`` header of value
+``Accept, Prefer, Range`` in its responses which should fit most of the bills. As any other
+response header, it's available for override
+by updating ``response.headers`` GUC variable accordingly, for example:
+
+.. code-block:: postgres
+
+   -- Override the Vary header to include Accept, Prefer and X-Test-Vary headers
+   perform set_config('response.headers', '[{"Vary": "Accept, Prefer, X-Test-Vary"}]', true);
+
+In this case PostgREST will use provided value verbatim.