Update README to show how environment-specific configuration files are loaded

Resolves #255

Author: nblumhardt

README.md

@@ -35,6 +35,7 @@ static void Main(string[] args)
     var configuration = new ConfigurationBuilder()
         .SetBasePath(Directory.GetCurrentDirectory())
         .AddJsonFile("appsettings.json")
+        .AddJsonFile($"appsettings.{Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT")}.json", true)
         .Build();
 
     var logger = new LoggerConfiguration()