docs(script): User will want to remove pacman's stale db.lck after rollback (#1)

README.md

@@ -74,6 +74,27 @@ Severity based on affected packages, here trivial ───────┘
 
 Have a look at `pacman-zfs-snapshot.conf` as well, its comments should be clear enough to get you going.
 
+# Rollback
+
+After a rollback for example via the excellent [ZFSBootMenu](https://docs.zfsbootmenu.org/) `pacman` and all AUR helpers you may be using will consider the `pacman` database to be locked. No `pacman` transactions can start, you will for example see:
+
+- In `pacman`
+    ```
+    # pacman -Syu
+    :: Synchronizing package databases...
+    error: failed to synchronize all databases (unable to lock database)
+    ```
+- In `paru`
+    ```
+    $ paru
+    :: Pacman is currently in use, please wait...
+    ```
+
+The moment a snapshot was created `pacman` was already in a transaction so it had already written its lock file to `/var/lib/pacman/db.lck`. After a clean finish `pacman` would have deleted that lock itself but since you rolled back to a point mid-transaction it's still there. Just delete the file and you're good to go:
+```
+sudo rm /var/lib/pacman/db.lck
+```
+
 # Development
 
 ## Conventional commits