(docs) - propose way for thirth party exchanges

docs/README.md

@@ -28,6 +28,11 @@ Everything about the basic & default `urql` behaviour
 that comes with explanations of the `fetchExchange`,
 the `cacheExchange`, and how to use _Subscriptions_.
 
+### [Exchanges](exchanges.md)
+
+Exchanges written by the community, these enhance
+the behavior of your urql-client.
+
 ### [Extending & Experimenting](extending-and-experimenting.md)
 
 Here's everything you need to know to extend,

docs/exchanges.md

@@ -0,0 +1,22 @@
+# Exchanges
+
+## Persisted Queries
+
+[Repository](https://github.com/Daniel15/urql-persisted-queries)
+
+Quoting from the repository:
+
+Apollo Server implements Automatic Persisted Queries (APQ),
+a technique that greatly improves network performance for GraphQL with
+zero build-time configuration. A persisted query is a ID or hash that
+can be sent to the server instead of the entire GraphQL query string.
+This smaller signature reduces bandwidth utilization and speeds up client
+loading times. Persisted queries are especially nice paired with GET requests,
+enabling the browser cache and integration with a CDN.
+
+With Automatic Persisted Queries, the ID is a deterministic hash of the input query,
+so we don't need a complex build step to share the ID between clients and servers.
+If a server doesn't know about a given hash, the client can expand the query for it.
+Apollo Server caches that mapping.
+
+Needless to say your server needs to support this.