chore: add README.md to nuget package

Author: guitarrapc

Directory.Build.props

@@ -1,22 +1,27 @@
 <Project>
-    <PropertyGroup>
-        <LangVersion>latest</LangVersion>
-        <Nullable>enable</Nullable>
-        <GenerateDocumentationFile>true</GenerateDocumentationFile>
-        <NoWarn>$(NoWarn);CS1591</NoWarn>
-        <SignAssembly>true</SignAssembly>
-        <AssemblyOriginatorKeyFile>$(MSBuildThisFileDirectory)opensource.snk</AssemblyOriginatorKeyFile>
+  <PropertyGroup>
+    <LangVersion>latest</LangVersion>
+    <Nullable>enable</Nullable>
+    <GenerateDocumentationFile>true</GenerateDocumentationFile>
+    <NoWarn>$(NoWarn);CS1591</NoWarn>
+    <SignAssembly>true</SignAssembly>
+    <AssemblyOriginatorKeyFile>$(MSBuildThisFileDirectory)opensource.snk</AssemblyOriginatorKeyFile>
 
-        <!-- NuGet Packaging -->
-        <PackageVersion>$(Version)</PackageVersion>
-        <Company>Cysharp</Company>
-        <Authors>Cysharp</Authors>
-        <Copyright>© Cysharp, Inc.</Copyright>
-        <PackageProjectUrl>https://github.com/Cysharp/MemoryPack</PackageProjectUrl>
-        <RepositoryUrl>$(PackageProjectUrl)</RepositoryUrl>
-        <RepositoryType>git</RepositoryType>
-        <PackageLicenseExpression>MIT</PackageLicenseExpression>
-        <PackageIcon>Icon.png</PackageIcon>
-        <AssemblyOriginatorKeyFile>$(MSBuildThisFileDirectory)opensource.snk</AssemblyOriginatorKeyFile>
-    </PropertyGroup>
-</Project>
+    <!-- NuGet Packaging -->
+    <PackageVersion>$(Version)</PackageVersion>
+    <Company>Cysharp</Company>
+    <Authors>Cysharp</Authors>
+    <Copyright>© Cysharp, Inc.</Copyright>
+    <PackageProjectUrl>https://github.com/Cysharp/MemoryPack</PackageProjectUrl>
+    <PackageReadmeFile>README.md</PackageReadmeFile>
+    <RepositoryUrl>$(PackageProjectUrl)</RepositoryUrl>
+    <RepositoryType>git</RepositoryType>
+    <PackageLicenseExpression>MIT</PackageLicenseExpression>
+    <PackageIcon>Icon.png</PackageIcon>
+    <AssemblyOriginatorKeyFile>$(MSBuildThisFileDirectory)opensource.snk</AssemblyOriginatorKeyFile>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <None Include="$(MSBuildThisFileDirectory)README.md" Pack="true" PackagePath="\" />
+  </ItemGroup>
+</Project>