add basic subscription doc

docs/subscriptions.md

@@ -0,0 +1,89 @@
+---
+id: subscriptions
+title: Subscriptions
+---
+
+Executing a GraphQL Subscription with Ferry is similar to queries, but with a few key differences:
+
+1. Setting up a WebSocket link for subscriptions
+2. Listening to the returned Stream for real-time updates
+
+
+## Setting up WebSocket Link
+
+First, you need to set up a WebSocket link for subscriptions. You can use the `gql_websocket_link` package for this. To add it to your project, run:
+(if using apollo use the TransportWebSocketLink)
+
+```yaml
+dependencies:
+  gql_websocket_link:
+```
+
+create the link:
+
+```dart
+Link.split(
+    (request) =>
+    request.operation.getOperationType() == OperationType.subscription,
+        TransportWebSocketLink(
+          TransportWsClientOptions(
+              socketMaker: WebSocketMaker.url(() => 'ws://<your url>/graphql'),
+              connectionParams: () => 
+              // add your auth headers here
+              webSocketHeaders),
+        ),
+        DioLink('https://<your url>/graphql', client: dio, defaultHeaders: headers),
+      )
+```
+
+
+## Creating the GraphQL Subscription Request
+
+Let's say we have a `NewReviews` Subscription saved in a file named `new_reviews.graphql`:
+
+```graphql
+subscription NewReviews {
+  newReviews {
+    id
+    title
+    content
+    author {
+      id
+      name
+    }
+  }
+}
+```
+
+after the file is created run your codegen command.
+
+## Using the Subscription
+
+Now that we have our subscription set up, we can use it in our Dart code. Here's an example of how to use the `NewReviews` subscription:
+
+```dart
+Stream<NewReviews?> getNewReviews() {
+  final request = GNewReviewsReq();
+
+  try {
+    final resultStream = _ferryService.request<GNewReviewsData, GNewReviewsVars>(request);
+    return resultStream.asyncMap((response) {
+      final data = response.data?.newReviews;
+      if (data != null) {
+        return NewReviews(
+          id: data.id,
+          title: data.title,
+          content: data.content,
+          author: data.author,
+        );
+      }
+      return null;
+    });
+  } catch (e) {
+    print('Failed to get new reviews: $e');
+    rethrow;
+  }
+}
+```
+
+