fix(github): Add a security info to the PR template

Signed-off-by: Joas Schilling <coding@schilljs.com>

.github/pull_request_template.md

@@ -1,3 +1,8 @@
+<!--
+  - 🚨 SECURITY INFO
+  -
+  - Before sending a pull request that fixes a security issue please report it via our HackerOne page (https://hackerone.com/nextcloud) following our security policy (https://nextcloud.com/security/). This allows us to coordinate the fix and release without potentially exposing all Nextcloud servers and users in the meantime.
+-->
 
 * Resolves: # <!-- related github issue -->