Add warning about different hashes

I think it is better to warn reader that commit/tag hashes in 10th chapter
are different from what he/she will get. Other hashes (of blobs and trees)
are identical so reader can think that he has done something wrong

Author: hedrok

book/10-git-internals/sections/objects.asc

@@ -242,6 +242,8 @@ $ echo 'first commit' | git commit-tree d8329f
 fdf4fc3344e67ab068f836878b6c4951e3b15f3d
 ----
 
+You will get another hash value because of different creation time and author data.
+Replace commit and tag hashes with your own checksums further in this chapter.
 Now you can look at your new commit object with `cat-file`:
 
 [source,console]