add hacktoberfest disclaimer to pr template (#3302)

Author: alexkrolick

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,21 @@
+Note to Hacktoberfest 🎃 participants:
 
+While we appreciate the enthusiasm, we are experiencing a high volume
+of drive-by pull requests (one-line changes, README tweaks, etc.).
+Please remember that hundreds of people are subscribed to this repo and
+will receive notifications for these PRs.
+Spam submissions will be closed and won't count towards your Hacktoberfest goals.
 
+Please search for issues tagged
+[`good first issue`][gfi]
+or [`hacktoberfest`][hacktoberfest]
+to find things to work on.
+
+You can also search [all of GitHub][all].
+
+[gfi]: https://github.com/reactjs/reactjs.org/issues?q=is%3Aissue+is%3Aopen+label%3A"good+first+issue"
+[hacktoberfest]: https://github.com/reactjs/reactjs.org/issues?q=is%3Aissue+is%3Aopen+label%3A"good+first+issue"
+[all]: https://github.com/search?q=is%3Aissue+hacktoberfest
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