Document GitLab Custom Hook environment variables

Zendesk: https://gitlab.zendesk.com/agent/tickets/123032

Problem to solve

We appear to export some environment variables for custom hooks. See https://gitlab.com/gitlab-org/gitlab-shell/blob/master/lib/gitlab_shell.rb#L173 for Git over SSH (GitLab Shell invocation directly) and https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/workhorse.rb#L30 for Git over HTTPS.

We should document these variables (GL_REPOSITORY, GL_PROJECT_PATH, and whatever others are available).

The Custom Hooks docs can be found at https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/administration/custom_hooks.md.