Added release notes for v2.12.0.

Author: JamesKovacs

Release Notes/Release Notes v2.12.0.md

@@ -1,3 +1,29 @@
 # .NET Driver Version 2.12.0 Release Notes
 
-This file is a placeholder for a future release.
+This is the general availability release for the 2.12.0 version of the driver.
+
+The main new features in 2.12.0 include:
+
+* Support for Hidden Indexes in MongoDB 4.4
+* Support for AWS temporary credentials in client-side field level encryption (CSFLE)
+* Support for Azure and GCP keystores in client-side field level encryption (CSFLE)
+* Support for client-side field level encryption (CSFLE) on Linux and Mac OSX
+* Support for GSSAPI/Kerberos on Linux
+* Support for .NET Standard 2.1
+* Various improvements in serialization performance
+* Fixed DNS failures in Kubernetes and Windows Subsystem for Linux (WSL/WSL2)
+* Fixed memory leak in heartbeat when cluster is inaccessible
+* Fixed SDAM deadlock when invalidating former primary
+
+An online version of these release notes is available at:
+
+https://github.com/mongodb/mongo-csharp-driver/blob/master/Release%20Notes/Release%20Notes%20v2.12.0.md
+
+The full list of JIRA issues that are currently scheduled to be resolved in this release is available at:
+
+https://jira.mongodb.org/issues/?jql=project%20%3D%20CSHARP%20AND%20fixVersion%20%3D%202.12.0%20ORDER%20BY%20key%20ASC
+
+Documentation on the .NET driver can be found at:
+
+https://docs.mongodb.com/drivers/csharp/
+