add top level dependency description

Directory.Build.targets

@@ -2,6 +2,8 @@
 <Project>
   <Import Project="Sdk.targets" Sdk="Microsoft.DotNet.Arcade.Sdk" />
   <ItemGroup>
+    <!-- This is to for the version of System.Security.Cryptography.X509Certificates to always be resolved
+        as 4.3 or higher. If this repo ever moves to central package version management, this can be removed. -->
     <PackageReference Include="System.Security.Cryptography.X509Certificates" Version="4.3.0" PrivateAssets="all" />
   </ItemGroup>
 </Project>