Add GitHub issue templates and contact links

Author: nosan

.github/ISSUE_TEMPLATE/config.yml

@@ -3,3 +3,6 @@ contact_links:
   - name: Asking for help
     url: https://stackoverflow.com/tags/spring-boot
     about: The Spring Boot team is using StackOverflow for general questions.
+  - name: Report a security vulnerability
+    url: https://github.com/spring-projects/spring-boot/security/policy
+    about: Please review our security policy for more details.