Add migration guide for Spectre.Cli

docs/input/cli/migration.md

@@ -0,0 +1,47 @@
+Title: Migrate from Spectre.Cli
+Order: 2
+---
+
+The functionality in `Spectre.Cli` has been moved into the `Spectre.Console`
+library. If you're using `Spectre.Cli`, you will need to migrate to ensure
+that you get updates or fixes.
+
+## 1. Remove Spectre.Cli NuGet package
+
+Start with removing the `Spectre.Cli` package reference from your project(s).
+
+```text
+> dotnet remove package Spectre.Cli
+```
+
+## 2. Add Spectre.Console NuGet package
+
+Add the [Spectre.Console](https://www.nuget.org/packages/spectre.console) NuGet package to your project(s).
+
+```text
+> dotnet add package Spectre.Console
+```
+
+## 3. Change using statements
+
+Change all using statements from `Spectre.Cli` 
+to `Spectre.Console.CLi`.
+
+```diff
+- using Spectre.Cli;
++ using Spectre.Console.Cli;
+```
+
+# Breaking Changes
+
+In the process of moving `Spectre.Cli`, there have been some minor breaking changes.
+
+## Spectre.Cli.Exceptions namespace moved
+
+All exceptions have been moved from the `Spectre.Cli.Exceptions` namespace to
+  the `Spectre.Console.Cli` namespace.
+
+```diff
+- using Spectre.Cli.Exceptions;
++ using Spectre.Console.Cli;
+```