Add some helper text for magical ADD
Closes:
@@ -643,6 +643,14 @@ guide](../articles/dockerfile_best-practices.md#build-cache) for more informatio
2. The contents of the source tree, with conflicts resolved in favor
of "2." on a file-by-file basis.
+ > **Note**:
+ > Whether a file is identified as a recognized compression format or not
+ > is done soley based on the contents of the file, not the name of the file.
+ > For example, if an empty file happens to end with `.tar.gz` this will not
+ > be recognized as a compressed file and **will not** generate any kind of
+ > decompression error message, rather the file will simply be copied to the
+ > destination.
+
- If `<src>` is any other kind of file, it is copied individually along with
its metadata. In this case, if `<dest>` ends with a trailing slash `/`, it
will be considered a directory and the contents of `<src>` will be written
