Added netstandard1.6 as target

Author: xperiandri

FSharp.Collections.Immutable.sln

@@ -1,5 +1,4 @@
-
-Microsoft Visual Studio Solution File, Format Version 12.00
+Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio Version 16
 VisualStudioVersion = 16.0.29021.104
 MinimumVisualStudioVersion = 10.0.40219.1

src/FSharp.Collections.Immutable/FSharp.Collections.Immutable.fsproj

@@ -1,17 +1,14 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFrameworks>netstandard1.6;netstandard2.0</TargetFrameworks>
     <Description>F# bindings for System.Collections.Immutable</Description>
     <Copyright>Copyright © XperiAndri 2016</Copyright>
     <AssemblyTitle>FSharp.Collections.Immutable</AssemblyTitle>
     <Company>XperiAndri</Company>
     <ProductName>FSharp.Collections.Immutable</ProductName>
-    <Copyright>Copyright © XperiAndri 2016</Copyright>
     <Version>1.0.0</Version>
-    <Authors>XperiAndri;EventHelix</Authors>
     <Authors>XperiAndri;EventHelix;vilinski</Authors>
-    <TargetFramework>netstandard2.0</TargetFramework>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <PackageId>FSharp.Collections.Immutable</PackageId>
     <PackageTags>System;Immutable;Collections;FSharp;F#</PackageTags>
@@ -30,8 +27,13 @@
     <Compile Include="sets.fs" />
   </ItemGroup>
 
+  <ItemGroup Condition="'$(TargetFramework)'=='netstandard1.6'">
+    <PackageReference Update="FSharp.Core" Version="4.6.2" />
+    <PackageReference Update="System.ValueTuple" Version="4.5.0" />
+  </ItemGroup>
+
   <ItemGroup>
-    <PackageReference Include="System.Collections.Immutable" Version="1.5.0" />
+    <PackageReference Include="System.Collections.Immutable" Version="1.7.0" />
   </ItemGroup>
 
 </Project>