Merge pull request #540 from CoolGoose/patch-1

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

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
 ```
 
 ### Yarn