Update docs for ASP.NET 5 / vNext support

[mikeckennedy]

As you may know, the exception filter stuff doesn't work the same in ASP.NET vNext as far as I can tell. The following steps work there instead. Consider updating the docs or adding a separate readme?

Create an ILogger:

public class RollbarExceptionLogger : ILogger
{
    public void Log(LogLevel logLevel, int eventId, object state, Exception exception, Func<object, Exception, string> formatter)
    {
        if (exception == null)
            return;

        const string accessToken = "YOUR_SERVER_ACCESS_TOKEN";
        RollbarClient rollbar = new RollbarClient(accessToken);
        rollbar.SendException(exception);
    }

    public bool IsEnabled(LogLevel logLevel)
    {
#if DEBUG
        return false;
#else
        return true;
#endif
    }

    public IDisposable BeginScopeImpl(object state)
    {
        return null;
    }
}

Next create a ILoggerProvider to generate them upon request.

public class RollbarExceptionLoggerProvider : ILoggerProvider
{
    public void Dispose()
    {
    }

    public ILogger CreateLogger(string categoryName)
    {
        return new RollbarExceptionLogger();
    }
}

Finally, install it in your Startup.cs's configure method call:

public class Startup
{
            public void Configure(IApplicationBuilder app, IHostingEnvironment env,
        ILoggerFactory loggerFactory, IOptions<MvcJsonOptions> mvcOptions )
    {
        Environment = env;
        mvcOptions.Value.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver();
        loggerFactory.AddConsole(Configuration.GetSection("Logging"));
        loggerFactory.AddProvider(new RollbarExceptionLoggerProvider());
        loggerFactory.AddDebug();
                   // ...
           }
    }