Request only genuine pull-requests are submitted

Update `PULL_REQUEST_TEMPLATE` with an attempt to persuade people not
to use our repo as a GitHub playground.

Author: philwebb

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,6 +2,9 @@
 Thanks for contributing to Spring Boot. Please review the following notes before
 submitting you pull request.
 
+Please submit only genuine pull-requests. Do not use this repository as a GitHub
+playground.
+
 Security Vulnerabilities
 
 STOP! If your contribution fixes a security vulnerability, please do not submit it.