Update README.md

Thought it will be good idea to include `JsonLowerCaseNamingPolicy` in example, started to look for it in google and then realized it's a custom policy. Maybe also good to include this naming policy in source code so folks don't need to create it? Let me know, happy to do it.

Author: robertlyson

README.md

@@ -25,6 +25,16 @@ jsonSerializerOptions.PropertyNamingPolicy = new JsonLowerCaseNamingPolicy();
 jsonSerializerOptions.AddProtobufSupport();
 ```
 
+```csharp
+private class JsonLowerCaseNamingPolicy : JsonNamingPolicy
+{
+  public override string ConvertName(string name)
+  {
+    return name.ToLower();
+  }
+}
+```
+
 You need to pass the options each time you want to serialize:
 
 ```csharp
@@ -73,4 +83,4 @@ Apple M1 Pro, 1 CPU, 10 logical and 10 physical cores
 
 ## License
 
-This project uses [MIT licence](https://github.com/Havret/Protobuf.System.Text.Json/blob/main/LICENSE). Long story short - you are more than welcome to use it anywhere you like, completely free of charge and without oppressive obligations.
+This project uses [MIT licence](https://github.com/Havret/Protobuf.System.Text.Json/blob/main/LICENSE). Long story short - you are more than welcome to use it anywhere you like, completely free of charge and without oppressive obligations.