Better comment the hooks

There seems to be some confusion over how to use hooks now that we symlinked the directory https://github.com/gitlabhq/gitlab-shell/issues/188

These comments should make things more clear.

hooks/post-receive

#!/usr/bin/env ruby
-# This file was placed here by GitLab. It makes sure that your pushed commits
-# will be processed properly.
-# You can add your own hooks to this file, but be careful when updating gitlab-shell!
+# This file was placed here by GitLab Shell.
+# It is a shared file that is symlinked from all repositories.
+# It makes sure that your pushed commits will be processed properly.
+# For instructions on how to add custom hooks see the comment below.
+# Consider using project services of post-receive hooks
+# See http://doc.gitlab.com/ce/integration/README.html
+# Or use web hooks that trigger an external application
+# See http://doc.gitlab.com/ce/web_hooks/web_hooks.html
changes = ARGF.read
key_id  = ENV['GL_ID']
@@ -11,6 +16,15 @@ repo_path = Dir.pwd
require_relative '../lib/gitlab_post_receive'
if GitlabPostReceive.new(repo_path, key_id, changes).exec
+  ## How to add a custom hooks to some repositories:
+  ## 1) Put a file with custom code in the repository root directory.
+  ## 2) Uncomment the lines below.
+  ## 3) Replace 'project-specific-hook' with the name if the custom file.
+  ## 4) If you upgrade GitLab Shell you will have to redo your changes.
+  #
+  # if File.exists?('project-specific-hook')
+  #   execute 'project-specific-hook'
+  # end
exit 0
else
exit 1