Updated Release Checklist (markdown)

Release-Checklist.md

@@ -5,6 +5,7 @@
 * If BC version bump: Check for disabled tests that are now fixed.
 * Check if code is in good shape -> `gradle check javadocAll`
 * Stable release? -> `git checkout -b release/X.Y`
+* Stable release? -> Update SECURITY.md
 * Set the version in `version.gradle`. Don't forget to set `isSnapshot = false`
 * `export PGPAINLESS_VERSION=X.Y.Z`
 * create commit: `git commit -a -S -m "PGPainless $PGPAINLESS_VERSION"`