docs(ecosystem): add fastify-json-to-xml plugin (fastify#4572)

Author: Fdawgs

docs/Guides/Ecosystem.md

@@ -376,6 +376,8 @@ section.
 - [`fastify-ip`](https://github.com/metcoder95/fastify-ip) A plugin
   for Fastify that allows you to infer a request ID by a
   given set of custom Request headers.
+- [`fastify-json-to-xml`](https://github.com/Fdawgs/fastify-json-to-xml) Fastify
+  plugin to serialize JSON responses into XML.
 - [`fastify-jwt-authz`](https://github.com/Ethan-Arrowood/fastify-jwt-authz) JWT
   user scope verifier.
 - [`fastify-jwt-webapp`](https://github.com/charlesread/fastify-jwt-webapp) JWT