add a Directory.Build.props which is sensible in context of this repo…

…sitory, and may help progress with appveyor and dotnet sdk update in this branch.

Directory.Build.props

@@ -0,0 +1,24 @@
+<Project>
+	<PropertyGroup Condition="'$(MSBuildProjectExtension)' == '.fsproj'">
+<!--
+  note: tolerate obsolete calls
+-->    
+    <WarningsNotAsErrors>$(WarningsNotAsErrors);44;</WarningsNotAsErrors>
+	  <LangVersion>latest</LangVersion>
+  </PropertyGroup>
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Release</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <Deterministic>true</Deterministic>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+<!--
+  note: preclude few nuget warnings of breaking the build
+  NU1504: Duplicate 'PackageReference' items found. Remove the duplicate items or use the Update functionality to ensure a consistent restore behavior. The duplicate 'PackageReference' items are: xxx 4.5.1, xxx 4.8.2.
+  NU1701: Package 'xxx 14.0.1016.290' was restored using 'yyy' instead of the project target framework 'zzz'. This package may not be fully compatible with your project.
+  NU1903: Package 'xxx' 2.0.0 has a known moderate severity vulnerability, https://github.com/advisories/xxx
+  NU1903: Package 'xxx' 2.0.0 has a known high severity vulnerability, https://github.com/advisories/xxx
+  NU1904: Package 'xxx' 5.0.0 has a known critical severity vulnerability, https://github.com/advisories/xxx
+-->
+    <WarningsNotAsErrors>$(WarningsNotAsErrors);NU1504;NU1701;NU1902;NU1903;NU1904</WarningsNotAsErrors>
+  </PropertyGroup>
+</Project>