Update README.md

Jiang-Xuan · web-flow · commit d30f22897f15 · 2017-03-24T17:22:21.000+08:00
highlight shell command:)
diff --git a/README.md b/README.md
@@ -41,7 +41,9 @@ NOTE: from log4js 0.5 onwards you'll need to explicitly enable replacement of no
 
 ## installation
 
+```shell
 npm install log4js
+```
 
 
 ## usage
