Add a note about the exact dir where to run `gdk install` command

Victor Zagorodnyrequested to merge
vzagorodny/gitlab-development-kit:vzagorodny-add-gdk-install-run-dir-note into master

Before, it was not so clear where to run the gdk install command after cloning the gitlab-ce or gitlab-ee repo and running gdk init. This addition tries to fix this problem.

Edited by Victor Zagorodny

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