Added release notes for v2.12.1.

Release Notes/Release Notes v2.12.1.md

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+# .NET Driver Version 2.12.1 Release Notes
+
+This is a patch release that addresses some issues reported since 2.12.0 was released.
+
+An online version of these release notes is available at:
+
+https://github.com/mongodb/mongo-csharp-driver/blob/master/Release%20Notes/Release%20Notes%20v2.12.1.md
+
+The list of JIRA tickets resolved in this release is available at:
+
+https://jira.mongodb.org/issues/?jql=project%20%3D%20CSHARP%20AND%20fixVersion%20%3D%202.12.1%20ORDER%20BY%20key%20ASC
+
+Documentation on the .NET driver can be found at:
+
+http://mongodb.github.io/mongo-csharp-driver/
+
+## Upgrading
+
+There are no known backwards breaking changes in this release.