add file license header.

src/DotNetCore.CAP.MongoDB/MongoTransaction.cs

-using System;
+// Copyright (c) .NET Core Community. All rights reserved.
+// Licensed under the MIT License. See License.txt in the project root for license information.
+
 using System.Threading.Tasks;
 using DotNetCore.CAP.Abstractions;
 using MongoDB.Driver;
@@ -8,6 +10,7 @@ namespace DotNetCore.CAP.MongoDB
     public class MongoTransaction : IMongoTransaction
     {
         private readonly IMongoClient _client;
+
         public MongoTransaction(IMongoClient client)
         {
             _client = client;

src/DotNetCore.CAP/Abstractions/IMongoTransaction.cs

+// Copyright (c) .NET Core Community. All rights reserved.
+// Licensed under the MIT License. See License.txt in the project root for license information.
+
 using System;
 using System.Threading.Tasks;