Add guide for dogfooding CoreFX

Documentation/project-docs/dogfooding.md

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+# How to patch dotnet CLI with latest CoreFX
+
+This document provides the steps necessary to consume a nightly build of CoreFX
+and CoreCLR.
+
+Please note that these steps aren't necessary for official builds -- these steps
+are specficic to consuming nightlies and thus unsupported builds. Also note that
+these steps are likely to change as we're simplifying this experience. Make
+sure to consult this document often.
+
+## Update framework and CLI
+
+1. Add the CoreFX feed by editing
+   `C:\Users\XXX\AppData\Roaming\NuGet\NuGet.Config` to add this line:
+    ```
+    <add key="CoreFX" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" />
+    ```
+	or, if you have `nuget.exe` in your path, this will work too:
+    ```
+	nuget sources Add "foo" https://dotnet.myget.org/F/dotnet-core/api/v3/index.json
+    ```
+
+2. Pick a package version from
+   <https://dotnet.myget.org/feed/dotnet-core/package/nuget/Microsoft.NETCore.App>
+   feed. Presumably you want the latest version.
+
+3. Get CLI preview4+:
+    1. Install CLI preview 4 SDK from here:
+        - <https://github.com/dotnet/cli>
+        - <https://dotnetcli.blob.core.windows.net/dotnet/Sdk/rel-1.0.0/dotnet-dev-win-x64.latest.exe>
+    2. Install 1.0.3 SDK from here:
+       - <https://go.microsoft.com/fwlink/?LinkID=836281>
+       - Currently necessary for the CLI itself to run.
+       - Eventually it will be chained-in by the SDK making this step
+         unncessary.
+       - The issue tracking this work is filed
+         [here](https://github.com/dotnet/cli/issues/5194).
+
+4. Verify that it works
+    1. Create a new app with `dotnet new` - don't reuse an old one, it must
+       match the SDK
+    2. In your app, add a call to some recently added API, to prove this works.
+    3. For example, this won't compile before .NET Core 1.2:
+    ```
+    System.Net.WebUtility.HtmlDecode("&amp;", Console.Out);
+    ```
+    4. Edit your `csproj` as explained below
+    5. Run `dotnet restore`
+    6. Run `dotnet run`
+
+10. Optionally, `dotnet publish` to make a standalone exe
+
+## Consuming the build
+
+In order to consume the latest build, you'll need to update your `.csproj`
+as follows:
+
+1. Update `TargetFramework`, add `RuntimeIdentifier` as below (ideally
+   `dotnet.exe` would infer your current architecture but it currently doesn't)
+2. Update package reference to match version selected above, as below
+
+```xml
+<Project Sdk="Microsoft.NET.Sdk" ToolsVersion="15.0">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp1.2</TargetFramework>
+    <RuntimeIdentifier>win10-x64</RuntimeIdentifier>
+  </PropertyGroup>
+
+  <!-- Make sure to use Update, not Include! -->
+  <ItemGroup>
+    <PackageReference Update="Microsoft.NETCore.App" Version="1.2.0-beta-001285-00" />
+  </ItemGroup>
+
+</Project>
+```