git-read-tree.txt: acknowledge the directory matching bug in sparse checkout

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-read-tree.txt

@@ -412,6 +412,13 @@ turn `core.sparseCheckout` on in order to have sparse checkout
 support.
 
 
+BUGS
+----
+In order to match a directory with $GIT_DIR/info/sparse-checkout,
+trailing slash must be used. The form without trailing slash, while
+works with .gitignore, does not work with sparse checkout.
+
+
 SEE ALSO
 --------
 linkgit:git-write-tree[1]; linkgit:git-ls-files[1];