Update CONTRIBUTING.md

Author: amitguptagwl

CONTRIBUTING.md

@@ -34,3 +34,9 @@ Note that publishing changes or accepting any PR may take time. So please keep p
 
 * https://github.com/Roshanjossey/first-contributions
 * **Don't stretch**. If you complete an issue in long time, there is a possibility that other developers finish their part and you face code conflicts which may increase code complexity for you. So it is always good to complete an issue ASAP. 
+* Please refrain to work on multiple issues marked with "first-timers-only" in the same repo. Ask and help your friends and colleagues to attempt rest issues.
+* Please claim the issue and clear your doubts before raising PR. So other users will not start working on the same issue.
+* Mention the issue number either in PR detail or in commit message.
+* Keep increasing the level of challenge.
+* Don't hesitate to question on github issue or on twitter.
+