Release notes for v2.21.0 (#1157)

Release Notes/Release Notes v2.21.0.md

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+# .NET Driver Version 2.21.0 Release Notes
+
+This is the general availability release for the 2.21.0 version of the driver.
+
+The main new features in 2.21.0 include:
+
+* LINQ3 bug fixes and improvements
+* Atlas Search sort, tracking, score details options added
+* Atlas Search embeddedDocuments operator support
+* Atlas Search index mangement API added
+* Anonymous types added to default AllowedTypes in ObjectSerializer
+
+### Atlas Search minor breaking change
+All ```Render``` methods in Atlas Search builders have a new signature:
+
+```Render(IBsonSerializer<TDocument> documentSerializer, IBsonSerializerRegistry serializerRegistry)```
+
+Changed to
+
+```Render(SearchDefinitionRenderContext<TDocument> renderContext)```
+
+An online version of these release notes is available [here](https://github.com/mongodb/mongo-csharp-driver/blob/master/Release%20Notes/Release%20Notes%20v2.21.0.md).
+
+The full list of issues resolved in this release is available at [CSHARP JIRA project](https://jira.mongodb.org/issues/?jql=project%20%3D%20CSHARP%20AND%20fixVersion%20%3D%202.21.0%20ORDER%20BY%20key%20ASC).
+
+Documentation on the .NET driver can be found [here](https://www.mongodb.com/docs/drivers/csharp/v2.21/).