docs(PullRequest): add no external contributions in PR template.

Author: arriolac

.github/pull_request_template.md

@@ -1,18 +1,7 @@
----
-name: Pull request
-about: Create a pull request
-label: 'triage me'
+**NOTE: We are not accepting external contributions at this time.**
 
----
+If you find a bug or have a feature request related to these samples, please [file an issue](https://github.com/googlemaps/maps-sdk-for-ios-samples/issues).
 
-Thank you for opening a Pull Request! 
-
----
-
-Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
-- [ ] Make sure to open a GitHub issue as a bug/feature request before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
-- [ ] Ensure the tests and linter pass
-- [ ] Code coverage does not decrease (if any source code was changed)
-- [ ] Appropriate docs were updated (if necessary)
-
-Fixes #<issue_number_goes_here> 🦕
+If you find a bug or have a feature request related to one of the SDKs, you can file an issue on either the
+[Google Maps SDK for iOS Issue Tracker](https://developers.google.com/maps/documentation/ios-sdk/support#issue-tracker).or the
+[Places SDK for iOS Issue Tracker](https://issuetracker.google.com/savedsearches/5050150).