Add README to NuGet packages and clean up branches

- Added PackageReadmeFile to Directory.Build.props so all NuGet
  packages include README.md (fixes "missing README" on nuget.org)
- README.md is packed from the repo root into every .nupkg

Author: imperugo

Directory.Build.props

@@ -14,6 +14,7 @@
         <SymbolPackageFormat>snupkg</SymbolPackageFormat>
         <PublishRepositoryUrl>true</PublishRepositoryUrl>
         <NoDefaultExcludes>true</NoDefaultExcludes>
+        <PackageReadmeFile>README.md</PackageReadmeFile>
         <Copyright>Ugo Lattanzi</Copyright>
         <NeutralLanguage>en-US</NeutralLanguage>
         <GenerateDocumentationFile>true</GenerateDocumentationFile>
@@ -92,6 +93,10 @@ v12.0.0:
         </AssemblyAttribute>
     </ItemGroup>
 
+    <ItemGroup>
+        <None Include="$(MSBuildThisFileDirectory)README.md" Pack="true" PackagePath="\" />
+    </ItemGroup>
+
     <ItemGroup>
         <AssemblyAttribute Include="System.Diagnostics.CodeAnalysis.SuppressMessage">
             <_Parameter1>Style</_Parameter1>