RELNOTES: add security-related items

Note: netblue30#4780 was closed by commit d2e10f8 ("fix --private-cwd problem",
2021-12-19).

Relates to netblue30#4748 netblue30#4780.

RELNOTES

@@ -1,5 +1,10 @@
 firejail (0.9.68rc2) baseline; urgency=low
   * work in progress
+  * security: the firejail package on Ubuntu was left vulnerable to
+    CVE-2021-26910 for months after a patch for it was made available; the PPA
+    is now recommended instead (#4748)
+  * security: bugfix: private-cwd leaks access to the entire filesystem
+    (#4780); reported by Hugo Osvaldo Barrera
   * exit code: distinguish fatal signals by adding 128 (#4533)
   * close file descriptors greater than 2 (--keep-fd) (#4845)
   * intrusion detection system (--ids-init, --ids-check)