Added snippets for the subtraction operator article (#929)

Author: pkulikov

csharp/language-reference/operators/Program.cs

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+using System;
+
+namespace operators
+{
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            Console.WriteLine("============== - operator examples =============");
+            SubtractionOperator.Examples();
+            Console.WriteLine();
+        }
+    }
+}

csharp/language-reference/operators/SubtractionOperator.cs

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+using System;
+
+namespace operators
+{
+    public static class SubtractionOperator
+    {
+        public static void Examples()
+        {
+            DelegateRemoval();
+            SubtractAndAssign();
+        }
+
+        private static void DelegateRemoval()
+        {
+            // <SnippetDelegateRemoval>
+            Action a = () => Console.Write("a");
+            Action b = () => Console.Write("b");
+
+            var abbaab = a + b + b + a + a + b;
+            abbaab();  // output: abbaab
+            Console.WriteLine();
+
+            var ab = a + b;
+            var abba = abbaab - ab;
+            abba();  // output: abba
+            Console.WriteLine();
+
+            var aba = a + b + a;
+            var first = abbaab - aba;
+            first();  // output: abbaab
+            Console.WriteLine();
+
+            var nihil = abbaab - abbaab;
+            Console.WriteLine(nihil is null);  // output: True
+            // </SnippetDelegateRemoval>
+        }
+
+        private static void SubtractAndAssign()
+        {
+            // <SnippetSubtractAndAssign>
+            int i = 5;
+            i -= 9;
+            Console.WriteLine(i);
+            // Output: -4
+
+            Action a = () => Console.Write("a");
+            Action b = () => Console.Write("b");
+            var printer = a + b + a;
+            printer();  // output: aba
+
+            Console.WriteLine();
+            printer -= a;
+            printer();  // output: ab
+            // </SnippetSubtractAndAssign>
+            Console.WriteLine();
+        }
+    }
+}

csharp/language-reference/operators/operators.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp2.2</TargetFramework>
+  </PropertyGroup>
+
+  <PropertyGroup>
+    <LangVersion>7.3</LangVersion>
+    <StartupObject>operators.Program</StartupObject>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+  </PropertyGroup>
+
+</Project>