Update CONTRIBUTING.md

Author: smv1999

CONTRIBUTING.md

@@ -4,6 +4,7 @@ Developers are highly welcome to contribute to this project by modifying the exi
 
 * Look at the issues created already and get yourself assigned for the issues by commenting there. Issues will be assigned on FCFS basis. 
 * Create a new issue whether you are adding a new program or optimizing the existing solution. After the issue is assigned to you, start working on it and make a pull request as mentioned in the steps below.
+* When creating a PR, make sure to add the linked issue in the description. For example : "Closes #133"
 
 1. Fork this repository
 1. Clone the repo ```git clone <YOUR_FORKED_REPO_URL>```