Write documentation for import existing git repositories to gerrit

The documentation should have a "quick start" section (plus all the stuff related to the configuration of Tuleap <-> Gerrit integration should be moved in the administration guide).

There are some opcion for import existing git repositories to gerrit.

1. AFTER of migrate
Create a Git repo inside your Tuleap project
Push your history into this new repo.
Migrate the repo to Gerrit.

2. BEFORE of migrate
Create a Git repo inside your Tuleap project
Migrate the repo to Gerrit
Push your history into this new repo.

If choose "2. BEFORE of migrate"
https://gerrit-review.googlesource.com/Documentation/install-quick.html#_already_existing_project

Another thing for add to documentation is the question of beginner AFTER of migrate:
Permisos vs No permissions
May be is good write about this ( how see proyect.config of git )