Move Windows .NET Core build instructions

docs/building/windows-core.md

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+Build PowerShell on Windows for .NET Core
+=========================================
+
+This guide will walk you through building PowerShell on Windows,
+targetting .NET Core. We'll start by showing how to set up your
+environment from scratch.
+
+Environment
+===========
+
+These instructions are tested on Windows 10 and Windows Server 2012
+R2, though they should work anywhere the dependencies work.
+
+.NET CLI
+--------
+
+We use the [.NET Command Line Interface][dotnet-cli] (`dotnet`) to
+build PowerShell. The following script will install `dotnet` and add
+it to your PowerShell session's path:
+
+```powershell
+Invoke-WebRequest -Uri https://raw.githubusercontent.com/dotnet/cli/rel/1.0.0/scripts/obtain/install.ps1 -OutFile install.ps1
+./install.ps1 -version 1.0.0.001888
+$env:Path += ";$env:LocalAppData\Microsoft\dotnet\cli
+```
+
+If you have any problems installing `dotnet`, please see their
+[documentation][cli-docs].
+
+If you are using Windows 7, Windows Server 2008 or Windows Server 2012
+you will also need to install
+[Visual C++ Redistributable for Visual Studio 2012 Update 4][redist-2012]
+and [Visual C++ Redistributable for Visual Studio 2015][redist-2015].
+
+The version of .NET CLI is very important, you want a recent build of
+1.0.0 (**not** 1.0.1).
+
+Previous installations of DNX, `dnvm`, or older installations of .NET
+CLI can cause odd failures when running. Please check your version.
+
+[dotnet-cli]: https://github.com/dotnet/cli#new-to-net-cli
+[cli-docs]: https://dotnet.github.io/getting-started/
+[redist-2012]: https://www.microsoft.com/en-us/download/confirmation.aspx?id=30679
+[redist-2015]: https://www.microsoft.com/en-us/download/details.aspx?id=48145
+
+Git Setup
+---------
+
+Please clone the superproject (this repo) and initialize a subset of
+the submodules:
+
+```sh
+git clone https://github.com/PowerShell/PowerShell.git
+cd PowerShell
+git submodule update --init -- src/windows-build src/Microsoft.PowerShell.Linux.Host/Modules/Pester
+```
+
+Build using our module
+======================
+
+We maintain a `PowerShellGitHubDev.psm1` PowerShell module with the
+function `Start-PSBuild` to build PowerShell.
+
+```powershell
+Import-Module ./PowerShellGitHubDev.psm1
+Start-PSBuild
+```
+
+Congratulations! If everything went right, PowerShell is now built and
+executable as `./bin/powershell.exe`.
+
+> The cross-platform host has built-in documentation via `--help`.
+
+You can run our cross-platform Pester tests with `./bin/powershell.exe
+-c "Invoke-Pester test/powershell"`.