Provide a README for dotnet completions

custom-completions/dotnet/README.md

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+# .NET CLI completions
+
+Completions for the .NET CLI (`dotnet`), which comes with .NET SDK.
+.NET is, to quote the official documentation, an "open-source developer platform for building many different types of applications".
+
+For more information, see
+
+- [Installation instructions](https://learn.microsoft.com/en-us/dotnet/core/install/),
+- [CLI documentation](https://learn.microsoft.com/en-us/dotnet/core/tools/),
+- [General dotnet documentation](https://learn.microsoft.com/en-us/dotnet/fundamentals/).
+
+This plugin uses built-in `dotnet complete` command, which unfortunately does not provide comments for the completions.
+On the other hand, it is officially supported, and completions are always in sync with the installed .NET SDK.
+For hand-crafted completions, see ones generated from Fish: (../auto-generate/completions/dotnet.nu).