add xlog library. (#1250)

README.md

@@ -567,6 +567,7 @@ See [go-hardware](https://github.com/rakyll/go-hardware) for a comprehensive lis
 * [slog](https://github.com/ventu-io/slog) - The reference implementation of the Structured Logging Facade (SLF) for Go
 * [stdlog](https://github.com/alexcesaro/log) - Stdlog is an object-oriented library providing leveled logging. It is very useful for cron jobs.
 * [tail](https://github.com/hpcloud/tail) - A Go package striving to emulate the features of the BSD tail program.
+* [xlog](https://github.com/xfxdev/xlog) - Plugin architecture and flexible log system for Go, with level ctrl, multiple log target and custom log format.
 * [xlog](https://github.com/rs/xlog) - A structured logger for `net/context` aware HTTP handlers with flexible dispatching.
 * [zap](https://github.com/uber-go/zap) - Fast, structured, leveled logging in Go.