Add a project file for .NET Standard 2.0

Source/Managed/ENet-CSharp.csproj

@@ -0,0 +1,21 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Library</OutputType>
+    <TargetFramework>netstandard2.0</TargetFramework>
+    <RootNamespace>ENet</RootNamespace>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
+	  <Optimize>false</Optimize>
+    <CheckForOverflowUnderflow>True</CheckForOverflowUnderflow>
+    <LangVersion>3</LangVersion>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|AnyCPU'">
+	  <Optimize>true</Optimize>
+    <CheckForOverflowUnderflow>True</CheckForOverflowUnderflow>
+    <LangVersion>3</LangVersion>
+  </PropertyGroup>
+
+</Project>