chore: Require contributors to open an issue first for opening a feature request

* Require contributors to open an issue first for opening a feature request

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Co-authored-by: Caleb Pollman <[email protected]>

Author: joon-won

.github/PULL_REQUEST_TEMPLATE.md

@@ -12,7 +12,7 @@ the requirements below.
 
 #### Issue #, if available
 <!-- Also, please reference any associated PRs for documentation updates. -->
-
+<!-- For external contributions, provide the github issue the PR is addressing. If no github issue exists for the related changes, open a new issue in https://github.com/aws-amplify/amplify-js/issues. -->
 
 
 #### Description of how you validated changes