Added support for net461 (.NET Framework 4.6.1).

Author: konard

Platform.Threading.csproj

@@ -6,7 +6,8 @@
     <AssemblyTitle>Platform.Threading</AssemblyTitle>
     <VersionPrefix>0.0.5</VersionPrefix>
     <Authors>Konstantin Diachenko</Authors>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFrameworks Condition="'$(OS)' == 'Windows_NT'">netstandard2.0;net461</TargetFrameworks>
+    <TargetFramework Condition="'$(OS)' != 'Windows_NT'">netcoreapp2.0</TargetFramework>
     <AssemblyName>Platform.Threading</AssemblyName>
     <PackageId>Platform.Threading</PackageId>
     <PackageTags>LinksPlatform;Threading;ISynchronization;ISynchronizationExtensions;ReaderWriterLockSynchronization;Unsynchronization;ISynchronized;ConcurrentQueueExtensions;TaskExtensions;ThreadHelpers</PackageTags>