Added 2.17.1 release notes. (#858)

Release Notes/Release Notes v2.17.1.md

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+# .NET Driver Version 2.17.1 Release Notes
+
+This is a patch release that fixes a potential data corruption bug in `RewrapManyDataKey` when rotating encrypted data encryption keys backed by GCP or Azure key services.
+
+The following conditions will trigger this bug:
+
+- A GCP-backed or Azure-backed data encryption key being rewrapped requires fetching an access token for decryption of the data encryption key.
+
+The result of this bug is that the key material for all data encryption keys being rewrapped is replaced by new randomly generated material, destroying the original key material.
+
+To mitigate potential data corruption, upgrade to this version or higher before using `RewrapManyDataKey` to rotate Azure-backed or GCP-backed data encryption keys. A backup of the key vault collection should **always** be taken before key rotation.
+
+An online version of these release notes is available at:
+
+https://github.com/mongodb/mongo-csharp-driver/blob/master/Release%20Notes/Release%20Notes%20v2.17.1.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%202.17.1%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.