Docs: add "JuiceFS performance statistics watcher" to TOC (#722)

docs/en/README.md

@@ -48,6 +48,7 @@ JuiceFS can simply and conveniently connect massive cloud storage directly to bi
   - [POSIX Compatibility](posix_compatibility.md)
   - [JuiceFS cache management](cache_management.md)
   - [JuiceFS operations profiling](operations_profiling.md)
+  - [JuiceFS performance statistics watcher](stats_watcher.md)
   - [JuiceFS fault diagnosis and analysis](fault_diagnosis_and_analysis.md)
   - [JuiceFS metrics](p8s_metrics.md)
   - [FUSE mount options](fuse_mount_options.md)

docs/zh_cn/README.md

@@ -46,6 +46,7 @@ JuiceFS 是一款高性能 [POSIX](https://en.wikipedia.org/wiki/POSIX) 文件
   - [POSIX 兼容性](posix_compatibility.md)
   - [JuiceFS 缓存管理](cache_management.md)
   - [JuiceFS 性能诊断](operations_profiling.md)
+  - [JuiceFS 性能统计监控](stats_watcher.md)
   - [JuiceFS 故障诊断和分析](fault_diagnosis_and_analysis.md)
   - [JuiceFS 监控指标](p8s_metrics.md)
   - [FUSE 挂载选项](fuse_mount_options.md)