Switch NuGet to use a license expression to assist with SBOM analysis

The COPYING.txt file, which was ported from the original C++
implementation, appears to be the basic BSD-3-Clause license except
for the parts about the testdata. However, the testdata is not included
in the NuGet package so isn't relevant.

Update the NuGet copyright to match the copyright notice in COPYING.txt.

Also embed the README.md file in the package.

Fixes #92

Snappier/Snappier.csproj

@@ -11,7 +11,7 @@
     <Authors>btburnett3</Authors>
     <PackageTags>snappy;compression;fast;io</PackageTags>
     <!-- Copyright remains with Google since this is a direct port of the C++ source from https://github.com/google/snappy -->
-    <Copyright>Copyright 2011, Google Inc. All rights reserved.</Copyright>
+    <Copyright>Copyright 2011-2020, Google, Inc. and Snappier Authors</Copyright>
     <Description>
       A near-C++ performance implementation of the Snappy compression algorithm for .NET. Snappier is ported to C# directly
       from the official C++ implementation, with the addition of support for the framed stream format.
@@ -20,7 +20,8 @@
       by .NET Core. However, Snappier performs best in .NET Core 3.0 and later on little-endian x86/64 processors with the
       help of System.Runtime.Instrinsics.
     </Description>
-    <PackageLicenseFile>COPYING.txt</PackageLicenseFile>
+    <PackageLicenseExpression>BSD-3-Clause</PackageLicenseExpression>
+    <PackageReadmeFile>README.md</PackageReadmeFile>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <PublishRepositoryUrl>true</PublishRepositoryUrl>
     <EmbedUntrackedSources>true</EmbedUntrackedSources>
@@ -38,7 +39,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <None Include="..\COPYING.txt" Pack="true" PackagePath="$(PackageLicenseFile)" />
+    <None Include="..\README.md" Pack="true" PackagePath="$(PackageReadmeFile)" />
   </ItemGroup>
 
   <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">