docs: add --save-dev to npm install command (#1289)

Add --save-dev to npm install command in README.md

Author: Hadi Ghattas

README.md

@@ -94,7 +94,7 @@ Chai is an _assertion library_, similar to Node's built-in `assert`. It makes te
 
 `chai` is available on [npm](http://npmjs.org). To install it, type:
 
-    $ npm install chai
+    $ npm install --save-dev chai
 
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