Add v3.2.1 release notes. (#1620)

Release Notes/Release Notes v3.2.1.md

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+# .NET Driver Version 3.2.1 Release Notes
+
+This is a patch release that addresses some issues reported since 3.2.0 was released:
+- Fix potential leak with KMS retry mechanism - [CSHARP-5489](https://jira.mongodb.org/browse/CSHARP-5489)
+- Fix stack overflow exception on POCOs that represents tree-like structures - [CSHARP-5493](https://jira.mongodb.org/browse/CSHARP-5493)
+
+An online version of these release notes is available [here](https://github.com/mongodb/mongo-csharp-driver/blob/master/Release%20Notes/Release%20Notes%20v3.2.1.md).
+
+The list of JIRA tickets resolved in this release is available at [CSHARP JIRA project](https://jira.mongodb.org/issues/?jql=project%20%3D%20CSHARP%20AND%20fixVersion%20%3D%203.2.1%20ORDER%20BY%20key%20ASC).
+
+Documentation on the .NET driver can be found [here](https://www.mongodb.com/docs/drivers/csharp/v3.2/).
+
+## Upgrading
+
+There are no known backwards breaking changes in this release.