chore: add readme to packages (#753)

Author: fgreinacher

Directory.Build.props

@@ -13,6 +13,7 @@
     <PackageTags>testing</PackageTags>
     <PackageProjectUrl>https://github.com/System-IO-Abstractions/System.IO.Abstractions</PackageProjectUrl>
     <PackageLicenseExpression>MIT</PackageLicenseExpression>
+    <PackageReadmeFile>README.md</PackageReadmeFile>
     <DefineConstants Condition="'$(TargetFramework)' == 'net5.0' OR '$(TargetFramework)' == 'netcoreapp3.1' OR '$(TargetFramework)' == 'netstandard2.1'">$(DefineConstants);FEATURE_ASYNC_FILE;FEATURE_ENUMERATION_OPTIONS;FEATURE_ADVANCED_PATH_OPERATIONS;FEATURE_PATH_JOIN_WITH_SPAN</DefineConstants>
     <DefineConstants Condition="'$(TargetFramework)' == 'net5.0'">$(DefineConstants);FEATURE_FILE_MOVE_WITH_OVERWRITE</DefineConstants>
     <DefineConstants Condition="'$(TargetFramework)' == 'net5.0'">$(DefineConstants);FEATURE_SUPPORTED_OS_ATTRIBUTE</DefineConstants>