Add @discardableResult to addDocument

Author: Eskils

Firestore/Swift/Source/Codable/CollectionReference+WriteEncodable.swift

@@ -31,6 +31,7 @@ public extension CollectionReference {
   ///                 the client is offline, though local changes will be visible
   ///                 immediately.
   /// - Returns: A `DocumentReference` pointing to the newly created document.
+  @discardableResult
   func addDocument<T: Encodable>(from value: T,
                                  encoder: Firestore.Encoder = Firestore.Encoder(),
                                  completion: ((Error?) -> Void)? = nil) throws