chore: Add deprecation notice with link to dotnet-core repository (#230)

Author: jsonbailey

README.md

@@ -1,3 +1,7 @@
+# [DEPRECATED]
+
+Development has moved to [dotnet-core repository](https://github.com/launchdarkly/dotnet-core/tree/main/pkgs/sdk/server). This repository remains here to support legacy references to dotnet-server-sdk.
+
 # LaunchDarkly Server-Side SDK for .NET
 
 The LaunchDarkly Server-Side SDK for .NET is designed primarily for use in multi-user systems such as web servers and applications. It follows the server-side LaunchDarkly model for multi-user contexts. It is not intended for use in desktop and embedded systems applications.
@@ -82,7 +86,7 @@ We encourage pull requests and other contributions from the community. Check out
     * Roll out a new feature to a subset of your users (like a group of users who opt-in to a beta tester group), gathering feedback and bug reports from real-world use cases.
     * Gradually roll out a feature to an increasing percentage of users, and track the effect that the feature has on key metrics (for instance, how likely is a user to complete a purchase if they have feature A versus feature B?).
     * Turn off a feature that you realize is causing performance problems in production, without needing to re-deploy, or even restart the application with a changed configuration file.
-    * Grant access to certain features based on user attributes, like payment plan (eg: users on the ‘gold’ plan get access to more features than users in the ‘silver’ plan). Disable parts of your application to facilitate maintenance, without taking everything offline.
+    * Grant access to certain features based on user attributes, like payment plan (eg: users on the 'gold' plan get access to more features than users in the 'silver' plan). Disable parts of your application to facilitate maintenance, without taking everything offline.
 * LaunchDarkly provides feature flag SDKs for a wide variety of languages and technologies. Read [our documentation](https://docs.launchdarkly.com/sdk) for a complete list.
 * Explore LaunchDarkly
     * [launchdarkly.com](https://www.launchdarkly.com/ "LaunchDarkly Main Website") for more information