init: document dotfiles exclusion on template copy

Not just . and .., but any path that begins with dot is not copied
when copying the template directory to a new repository.  You can
customize the template directory, copying some dotfiles might make
sense, but it's actually a good thing not to, because you would not
want to have your git directory copied in every git directory that
is created should you decide to put your template directory under
version control, for example.  Plus, it might be used as a feature
by people who would want to exclude some files.

Signed-off-by: Grégoire Paris <postmaster@greg0ire.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Grégoire Paris

Documentation/git-init.txt

@@ -116,8 +116,8 @@ does not exist, it will be created.
 TEMPLATE DIRECTORY
 ------------------
 
-The template directory contains files and directories that will be copied to
-the `$GIT_DIR` after it is created.
+Files and directories in the template directory whose name do not start with a
+dot will be copied to the `$GIT_DIR` after it is created.
 
 The template directory will be one of the following (in order):