Add another common error

Author: sevanspowell

docs/code/Flakes/Reference/CommonErrors.md

@@ -8,3 +8,9 @@ error: getting status of '/nix/store/1dg75ahm07ah26phnd1jy1jq06s680ps-source/som
 ```
 
 Make sure the file is staged to version control. Flakes can't see files that are not tracked by version control.
+
+```
+warning: Git tree '/home/rodney/iohk/cardano-wallet' is dirty
+```
+
+It can be hidden (if desired) by using the allow-dirty = true option in nix.conf.