Add install instructions for root user (#3419)

* Add install instructions for root user

* Reword the comment

* Update README.md

Commit adds a reference to solve the EACCES error while installation.

* remove trailing whitespace

Author: zeekhuge

README.md

@@ -24,6 +24,8 @@ $ npm install -g parse-server mongodb-runner
 $ mongodb-runner start
 $ parse-server --appId APPLICATION_ID --masterKey MASTER_KEY --databaseURI mongodb://localhost/test
 ```
+***Note:*** *If installation with* `-g` *fails due to permission problems* (`npm ERR! code 'EACCES'`), *please refer to [this link](https://docs.npmjs.com/getting-started/fixing-npm-permissions).*
+ 
 
 ### Inside a Docker container
 ```