docs: nuget documentation (#2197)

Adds initial documentation on how .NET support works, and how Renovate knows which packages to update via nuget.org.

Closes #2156

website/docs/nuget.md

+---
+title: Nuget (.NET)
+description: Nuget (.NET) dependencies support in Renovate
+---
+
+# Nuget
+
+Renovate supports upgrading dependencies in `.csproj` files.
+
+## Version Support
+
+Only SDK-style `.csproj` files are currently supported. By default, this includes:
+
+- .NET Core 1.0 and above
+- .NET Standard class libraries
+- Any `.csproj` in the SDK-style syntax
+
+To convert your .NET Framework .csproj into an SDK-style project, one can follow the [following guide](https://natemcmaster.com/blog/2017/03/09/vs2015-to-vs2017-upgrade/).
+
+## How It Works
+
+1.  Renovate will search each repository for any files with a `.csproj` extension.
+2.  Existing dependencies will be extracted from `<PackageReference>` tags
+3.  Renovate will look up the latest version on [nuget.org](https://nuget.org) to determine if any upgrades are available
+4.  If the source package includes a GitHub URL as its source, and has either a "changelog" file or uses GitHub releases, then Release Notes for each version will be embedded in the generated PR.
+
+## Future work
+
+Contributions and/or feature requests are welcome to support more patterns or additional use cases.