Release notes for 2.19.0. (#1013)

* Release notes for 2.19.0.

Release Notes/Release Notes v2.19.0.md

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+# .NET Driver Version 2.19.0 Release Notes
+
+This is the general availability release for the 2.19.0 version of the driver.
+
+The main new features in 2.19.0 include:
+
+* Atlas Search builders
+* Default LinqProvider changed to LINQ3
+* Support for Range Indexes preview
+* ObjectSerializer allowed types configuration
+* Bucket and BucketAuto stages support in LINQ3
+* Support Azure VM-assigned Managed Identity for Automatic KMS Credentials
+* Native support for AWS IAM Roles
+
+### ObjectSerializer allowed types configuration
+
+The `ObjectSerializer` has been changed to only allow deserialization of types that are considered safe. 
+What types are considered safe is determined by a new configurable `AllowedTypes` function (of type `Func<Type, bool>`).
+The default `AllowedTypes` function is `ObjectSerializer.DefaultAllowedTypes` which returns true for a number of well-known framework types that we have deemed safe.
+A typical example might be to allow all the default allowed types as well as your own types. This could be accomplished as follows:
+
+```
+var objectSerializer = new ObjectSerializer(type => ObjectSerializer.DefaultAllowedTypes(type) || type.FullName.StartsWith("MyNamespace"));
+BsonSerializer.RegisterSerializer(objectSerializer);
+```
+
+More information about the `ObjectSerializer` is available in [our FAQ](https://www.mongodb.com/docs/drivers/csharp/v2.19/faq).
+
+### Default LinqProvider changed to LINQ3
+Default LinqProvider has been changed to LINQ3.
+LinqProvider can be changed back to LINQ2 in the following way:
+
+```
+var connectionString = "mongodb://localhost";
+var clientSettings = MongoClientSettings.FromConnectionString(connectionString);
+clientSettings.LinqProvider = LinqProvider.V2;
+var client = new MongoClient(clientSettings);
+```
+If you encounter a bug in LINQ3 provider, please report it in [CSHARP JIRA project](https://jira.mongodb.org/projects/CSHARP/issues).
+
+An online version of these release notes is available [here](https://github.com/mongodb/mongo-csharp-driver/blob/master/Release%20Notes/Release%20Notes%20v2.19.0.md).
+
+The full list of issues resolved in this release is available at [CSHARP JIRA project](https://jira.mongodb.org/issues/?jql=project%20%3D%20CSHARP%20AND%20fixVersion%20%3D%202.19.0%20ORDER%20BY%20key%20ASC).
+
+Documentation on the .NET driver can be found [here](https://www.mongodb.com/docs/drivers/csharp/v2.19/).