Update Package license/icon/readme to current standards

Author: Numpsy

assets/icon.png

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <Description>XML configuration (System.Configuration &lt;appSettings&gt;) support for Serilog.</Description>
@@ -16,9 +16,10 @@
     <PublicSign Condition=" '$(OS)' != 'Windows_NT' ">true</PublicSign>
     <PackageId>Serilog.Settings.AppSettings</PackageId>
     <PackageTags>serilog;xml</PackageTags>
-    <PackageIconUrl>https://serilog.net/images/serilog-configuration-nuget.png</PackageIconUrl>
+    <PackageIcon>icon.png</PackageIcon>
+    <PackageReadmeFile>README.md</PackageReadmeFile>
     <PackageProjectUrl>https://github.com/serilog/serilog-settings-appsettings</PackageProjectUrl>
-    <PackageLicenseUrl>https://www.apache.org/licenses/LICENSE-2.0</PackageLicenseUrl>
+    <PackageLicenseExpression>Apache-2.0</PackageLicenseExpression>
     <RepositoryUrl>https://github.com/serilog/serilog-settings-appsettings</RepositoryUrl>
     <RepositoryType>git</RepositoryType>
     <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
@@ -41,4 +42,9 @@
     <PackageReference Include="System.Configuration.ConfigurationManager" Version="4.4.1" />
   </ItemGroup>
 
+  <ItemGroup>
+    <None Include="..\..\assets\icon.png" Pack="true" PackagePath="" Visible="false" />
+    <None Include="..\..\README.md" Pack="true" PackagePath="" />
+  </ItemGroup>
+
 </Project>