chore(readme): add improved install information (reactstrap#1220)

* Update README.md

Update installation steps with package runner tool, which can avoid installing create-react-app globally,  that comes with npm 5.2+

README.md

@@ -12,13 +12,20 @@ Follow the [create-react-app instructions](https://github.com/facebook/create-re
 
 ### tl;dr
 
+ ```
+npx create-react-app my-app
+cd my-app/
+npm start
+```
+or,  if npx (Node >= 6 and npm >= 5.2 ) not available 
+
 ```
 npm install -g create-react-app
 
 create-react-app my-app
 cd my-app/
 npm start
-```
+``` 
 
 Then open [http://localhost:3000/](http://localhost:3000/) to see your app. The initial structure of your app is setup. Next, let's [add reactstrap and bootstrap](#adding-bootstrap).