Add Readme for dotnetsay (#1304)

.gitignore

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 project.lock.json
 .DS_Store
 *.pyc
+nupkg/
 
 # Visual Studio Code
 .vscode

samples/dotnetsay/README.md

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+# dotnetsay .NET Core Global Tools Sample
+
+This sample demonstrates how to use and create .NET Core Global Tools. It works on Windows, macOS and Linux.
+
+You must have [.NET Core 2.1 Preview 1](https://blogs.msdn.microsoft.com/dotnet/2018/02/27/announcing-net-core-2-1-preview-1/) or higher installed.
+
+## Try the pre-built `dotnetsay` Global Tool
+
+You can quickly try the `dotnetsay` global tool using the following commands:
+
+```console
+dotnet install tool -g dotnetsay
+dotnetsay
+```
+
+This command will install [dotnetsay from nuget.org](https://www.nuget.org/packages/dotnetsay/).
+
+> Note: You may need to open a new command/terminal window the first time you install a tool.
+
+## Getting the sample
+
+The easiest way to get the sample is by cloning the samples repository with [git](https://git-scm.com/downloads), using the following instructions.
+
+```console
+git clone https://github.com/dotnet/core/
+```
+
+You can also [download the repository as a zip](https://github.com/dotnet/core/archive/master.zip).
+
+## Build the Tool
+
+You can build and run the sample using the following commands. The instructions assume that you are in the root of the repository.
+
+```console
+cd samples
+cd dotnetsay
+dotnet pack -c release -o nupkg
+```
+
+The last command packs the tool as a NuGet package in the `nupkg` directory. You can host the package at [nuget.org](https://www.nuget.org/) or any other NuGet feed. The tool is ready to install and test.
+
+The `PackAsTool` property in the project file makes it a global tool, as you can see in the following example. That's all you need to add to a console app to make it a tool.
+
+```xml
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp2.1</TargetFramework>
+    <PackAsTool>true</PackAsTool>
+  </PropertyGroup>
+
+</Project>
+```
+
+## Install the Tool
+
+You can build and run the sample using the following command.
+
+```console
+dotnet install tool -g dotnetsay
+```
+
+For Preview 1, defining the source during installation doesn't work correctly, so you need a nuget.config file to test your new tool without deploying it to a NuGet feed. You can do this by placing this nuget.config in your project directory that looking similar to the followin example:
+
+```xml
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+ <packageSources>
+    <clear/>
+    <add key="local-packages" value="./nupkg" />
+ </packageSources>
+</configuration>
+```
+
+## Run the Tool
+
+You can install the sample using the following command.
+
+```console
+dotnet install tool -g dotnetsay
+```

samples/dotnetsay/nuget.config

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 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
  <packageSources>
+    <clear/>
     <add key="local-packages" value="./nupkg" />
  </packageSources>
 </configuration>