add cors headers to http proxy

docs/reference-configuration.adoc

@@ -401,6 +401,14 @@ See <<tls>> section
 If set to `true` then Dshackle preserves _batch_ order based on request order.
 Note that latter is ineffective and use this option only when a client cannot reference responses by their IDs.
 
+| `сors-origin`
+|
+| Access-Control-Allow-Origin contents. If empty the header will be omitted in response
+
+| `cors-allowed-headers`
+| `Content-Type`
+| Access-Control-Allow-Headers contents. Takes effect only if сors-origi is present in config
+
 | `routes`
 |
 a| Routing paths for Proxy.