Update PR template for new charts (helm#16669)

Signed-off-by: Rita Zhang <rita.z.zhang@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -24,6 +24,8 @@ will run across your changes and do some initial checks and linting. These check
 very quickly. Please check the results. We would like these checks to pass before we
 even continue reviewing your changes.
 -->
+#### Is this a new chart
+> NOTE: We're experiencing a high volume of PRs to this repo and reviews will be delayed. Please host your own chart repository and submit your repository to the Helm Hub instead of this repo to make them discoverable to the community. [Here](https://github.com/helm/hub/blob/master/Repositories.md) is how to submit new chart repositories to the Helm Hub.
 
 #### What this PR does / why we need it: