Add documentation about reusing contract resolvers (#1785)

Doc/ContractResolver.aml

@@ -14,6 +14,9 @@
       <para>Anything that can be set on an object, collection, property, etc, using attributes or methods to control serialization
       can also be set using an IContractResolver.</para>
       <autoOutline lead="none" excludeRelatedTopics="true" />
+<alert class="note">
+  <para>For performance you should create a contract resolver once and reuse instances when possible. Resolving contracts is slow and implementations of <codeEntityReference>T:Newtonsoft.Json.Serialization.IContractResolver</codeEntityReference> typically cache contracts.</para>
+</alert>
     </introduction>
     <!-- Add one or more top-level section elements.  These are collapsible.
          If using <autoOutline />, add an address attribute to identify it

Doc/Performance.aml

@@ -6,6 +6,20 @@
       Here are some tips to make it go even faster.</para>
       <autoOutline lead="none" excludeRelatedTopics="true" />
     </introduction>
+    <section address="ReuseContractResolver">
+      <title>Reuse Contract Resolver</title>
+      <content>
+        <para>The <codeEntityReference>T:Newtonsoft.Json.Serialization.IContractResolver</codeEntityReference> resolves .NET types to contracts that are used during serialization inside JsonSerializer.
+        Creating a contract involves inspecting a type with slow reflection, so contracts are typically
+        cached by implementations of IContractResolver like <codeEntityReference>T:Newtonsoft.Json.Serialization.DefaultContractResolver</codeEntityReference>.</para>
+        <para>To avoid the overhead of recreating contracts every time you use JsonSerializer you should create the contract resolver once and reuse it.
+        Note that if you are not using a contract resolver then a shared internal instance is automatically used when serializing and deserializing.
+        </para>
+
+<code lang="cs" source="..\Src\Newtonsoft.Json.Tests\Documentation\PerformanceTests.cs" region="ReuseContractResolver" title="Reuse ContractResolver" />
+        
+      </content>
+    </section>
     <section address="MemoryUsage">
       <title>Optimize Memory Usage</title>
       <content>

Src/Newtonsoft.Json.Tests/Documentation/PerformanceTests.cs

@@ -111,6 +111,42 @@ namespace Newtonsoft.Json.Tests.Documentation
     [TestFixture]
     public class PerformanceTests : TestFixtureBase
     {
+        private static class AppSettings
+        {
+            public static readonly IContractResolver SnakeCaseContractResolver = new DefaultContractResolver();
+        }
+
+        [Test]
+        public void ReuseContractResolverTest()
+        {
+            Person person = new Person();
+
+            #region ReuseContractResolver
+            // BAD - a new contract resolver is created each time, forcing slow reflection to be used
+            string json1 = JsonConvert.SerializeObject(person, new JsonSerializerSettings
+            {
+                Formatting = Formatting.Indented,
+                ContractResolver = new DefaultContractResolver
+                {
+                    NamingStrategy = new SnakeCaseNamingStrategy()
+                }
+            });
+
+            // GOOD - reuse the contract resolver from a shared location
+            string json2 = JsonConvert.SerializeObject(person, new JsonSerializerSettings
+            {
+                Formatting = Formatting.Indented,
+                ContractResolver = AppSettings.SnakeCaseContractResolver
+            });
+
+            // GOOD - an internal contract resolver is used
+            string json3 = JsonConvert.SerializeObject(person, new JsonSerializerSettings
+            {
+                Formatting = Formatting.Indented
+            });
+            #endregion
+        }
+
         [Test]
         public void ConverterContractResolverTest()
         {