feat(docs): note on st_mtime bug

.vscode/settings.json

@@ -9,6 +9,7 @@
   "cSpell.words": [
     "ddir",
     "pytest",
+    "shutil",
     "venv",
     "yannickkirschen"
   ],
@@ -25,4 +26,4 @@
   "[python]": {
     "editor.defaultFormatter": "ms-python.autopep8"
   }
-}
+}

README.md

@@ -10,6 +10,9 @@ resolve them.
 
 Check out the [diff file format](#the-diff-file-format) as well.
 
+> [!Caution]
+> When copying data from the internal storage device to an external one, there currently is a strange bug that causes the metadata to be incorrect. Precisely, `shutil.copy2` does not manage to copy `st_mtime` correctly - there is always a difference by a couple of milliseconds. I have no idea where this comes from. In [v2.0.2](https://github.com/yannickkirschen/ddir/releases/tag/2.0.2) I added ceiling of the timestamps but removed it in [v3.1.0](https://github.com/yannickkirschen/ddir/releases/tag/3.1.0) cause it was just a workaround. Still looking for a solution ...
+
 ## Installation
 
 `pip install ddir`