Added v2.12.0-beta01 release notes

Author: JamesKovacs

Release Notes/Release Notes v2.12.0-beta01.md

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+# .NET Driver Version 2.12.0-beta01 Release Notes
+
+This is a beta release for the 2.12.0 version of the driver.
+
+The main new features in 2.12.0-beta01 include:
+
+* Support for Hidden Indexes in MongoDB 4.4
+* Support for Azure and GCP keystores in client-side field level encryption (CSFLE)
+* Fixed memory leak in heartbeat when cluster is inaccessible
+
+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-beta01.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
+
+The list may change as we approach the release date.
+
+Documentation on the .NET driver can be found at:
+
+http://mongodb.github.io/mongo-csharp-driver/
+