Add net5.0 target; use (source-only) Nullable package instead of bundling down-level attributes (#228)

Author: nblumhardt

serilog-sinks-file.sln

@@ -11,7 +11,6 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "assets", "assets", "{E9D1B5
 		appveyor.yml = appveyor.yml
 		Build.ps1 = Build.ps1
 		build.sh = build.sh
-		NuGet.Config = NuGet.Config
 		README.md = README.md
 		assets\Serilog.snk = assets\Serilog.snk
 	EndProjectSection

src/Serilog.Sinks.File/Serilog.Sinks.File.csproj

@@ -4,7 +4,7 @@
     <Description>Write Serilog events to text files in plain or JSON format.</Description>
     <VersionPrefix>5.0.0</VersionPrefix>
     <Authors>Serilog Contributors</Authors>
-    <TargetFrameworks>net45;netstandard1.3;netstandard2.0;netstandard2.1</TargetFrameworks>
+    <TargetFrameworks>net45;netstandard1.3;netstandard2.0;netstandard2.1;net5.0</TargetFrameworks>
     <LangVersion>8.0</LangVersion>
     <Nullable>enable</Nullable>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
@@ -14,7 +14,7 @@
     <PackageTags>serilog;file</PackageTags>
     <PackageIcon>images\icon.png</PackageIcon>
     <PackageIconUrl>https://serilog.net/images/serilog-sink-nuget.png</PackageIconUrl>
-    <PackageProjectUrl>http://serilog.net</PackageProjectUrl>
+    <PackageProjectUrl>https://serilog.net</PackageProjectUrl>
     <PackageLicenseExpression>Apache-2.0</PackageLicenseExpression>
     <RepositoryUrl>https://github.com/serilog/serilog-sinks-file</RepositoryUrl>
     <RepositoryType>git</RepositoryType>
@@ -28,7 +28,8 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All" />
-    <PackageReference Include="Serilog" Version="2.9.0" />
+    <PackageReference Include="Serilog" Version="2.10.0" />
+    <PackageReference Include="Nullable" Version="1.3.0" />
   </ItemGroup>
 
   <PropertyGroup Condition=" '$(TargetFramework)' == 'net45' ">
@@ -42,5 +43,5 @@
   <ItemGroup>
     <None Include="..\..\assets\serilog-sink-nuget.png" Pack="true" Visible="false" PackagePath="images\icon.png" />
   </ItemGroup>
-  
+
 </Project>