add description what is gitlab-shell

doc/install/installation.md

@@ -92,8 +92,11 @@ Create a `git` user for Gitlab:
     sudo adduser --disabled-login --gecos 'GitLab' git
+
 # 4. GitLab shell
+GitLab Shell is a ssh access and repository management software developed specially for GitLab.
+
     # Login as git 
     sudo su git