Add readme to nuget package (#556)

Author: robertcoltheart

Directory.Build.props

@@ -9,10 +9,12 @@
     <RepositoryUrl>http://github.com/machine/machine.specifications</RepositoryUrl>
     <RepositoryType>git</RepositoryType>
     <PackageLicenseExpression>MIT</PackageLicenseExpression>
+    <PackageReadmeFile>README.md</PackageReadmeFile>
   </PropertyGroup>
-  
+
   <ItemGroup>
     <None Include="../../images/icon.png" Pack="true" PackagePath="" Visible="false" />
+    <None Include="../../README.md" Pack="true" PackagePath="/" />
   </ItemGroup>
 
 </Project>