Update contributing.md

contributing.md

@@ -12,4 +12,6 @@
 * **Submit one pull request with one link per PR.** If you want to add several links, you should create PR for each link, unless all of the proposed projects are belong to the same category. Pull request should have meaningful description what this project do, why do you want to add it to the list and any other info which might be useful. This will help maintainers with the review process.
 * **Include a link to the added project in the PR description.** This will greatly improve email messages received by all of the people watching this repository.
 
+> Notes: The repository in github or any source control should be created at least 3 months. 
+
 Thanks to all [contributors](https://github.com/thangchung/awesome-dotnet-core/graphs/contributors), you're awesome and wouldn't be possible without you!