Save dev is 90% of the time more usefull than nothing

CoolGoose · web-flow · commit 67094f4870cb · 2016-12-23T19:00:45.000+02:00
diff --git a/docs/en/installation.md b/docs/en/installation.md
@@ -18,7 +18,7 @@ Include `vuex` after Vue and it will install itself automatically:
 ### NPM
 
 ``` bash
-npm install vuex
+npm install vuex --save-dev
 ```
 
 ### Yarn
