Include Readme.md in packages (#57809)

* Include Readme.md in packages

* Fix

* Try this

* Fix syntax

* Remove bad ref

Author: wtgodbe

Directory.Build.props

@@ -239,6 +239,18 @@
     <ArchiveExtension Condition="'$(TargetOsName)' == 'win'">.zip</ArchiveExtension>
   </PropertyGroup>
 
+  <PropertyGroup>
+    <!-- PackageReadmeFile specifies the package readme file name in the package. PackageReadmeFilePath points to the package readme file on disk. -->
+    <EnableDefaultPackageReadmeFile Condition="'$(EnableDefaultPackageReadmeFile)' == '' and '$(IsShipping)' != 'false'">true</EnableDefaultPackageReadmeFile>
+    <PackageReadmeFilePath Condition="'$(PackageReadmeFilePath)' == '' and '$(EnableDefaultPackageReadmeFile)' == 'true'">PACKAGE.md</PackageReadmeFilePath>
+    <PackageReadmeFile Condition="'$(PackageReadmeFile)' == '' and '$(EnableDefaultPackageReadmeFile)' == 'true' and Exists('$(PackageReadmeFilePath)' )">PACKAGE.md</PackageReadmeFile>
+  </PropertyGroup>
+
+  <!-- Add a package README file -->
+  <ItemGroup Condition="'$(PackageReadmeFilePath)' != '' and Exists('$(PackageReadmeFilePath)' )">
+    <None Include="$(PackageReadmeFilePath)" Pack="true" PackagePath="\" />
+  </ItemGroup>
+
   <PropertyGroup>
     <!-- Set the arcade before common targets so we compute ExcludeFromBuild before arcade uses it. -->
     <CustomBeforeMicrosoftCommonTargets>$(MSBuildThisFileDirectory)Directory.Build.BeforeCommonTargets.targets</CustomBeforeMicrosoftCommonTargets>