Update RELEASE_NOTES.md

Mention backquote fix in release notes

Author: Bike

RELEASE_NOTES.md

@@ -8,6 +8,7 @@
 ## Fixed
 * Weak pointers and weak hash tables survive snapshot save/load.
 * `ext:quit` can be used from any thread, not just the initial thread.
+* Quasiquoted vectors are read correctly (#1666).
 
 ## Removed
 * `-z`/`--snapshot-symbols-save` command line option, occasionally used