Update C#-Operations Docs

[maaft]

Currently Enums are not properly serialized when the standard Newtonsoft Serializer is used.

Solution:

public sealed class GraphQLEnumConverter : StringEnumConverter
  {
    public GraphQLEnumConverter()
        : base(new PascalCaseNamingStrategy())
    {
    }
  }

  internal sealed class PascalCaseNamingStrategy : NamingStrategy
  {
    protected override string ResolvePropertyName(string name) => name;
  }

GQLClient = new GraphQLHttpClient(new GraphQLHttpClientOptions
      {
        EndPoint = new Uri(EndpointHttp)

      }, new NewtonsoftJsonSerializer(new JsonSerializerSettings
      {
        Converters = new List<JsonConverter>{
              new StringEnumConverter(new PascalCaseNamingStrategy())
          }
      }));

This should be added to the docs.

[ardatan]

@maaft Could you create a PR that adds this to the docs?