Update documentation

[martialblog]

Use shallow git clone to speed up download and reduce size on filesystem.

Before

git clone "${REPO_URL}" "${TARGET_DIR}" --branch v${MODULE_VERSION}
Cloning into '/usr/share/icingaweb2/modules/director'...
remote: Enumerating objects: 38601, done.

After

git clone --depth=1 "${REPO_URL}" "${TARGET_DIR}" --branch v${MODULE_VERSION}
Cloning into '/usr/share/icingaweb2/modules/director'...
remote: Enumerating objects: 1223, done.

Regards Markus

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[bobapple]

@cla-bot check

[Thomas-Gelf]

This currently downloads 3,09 MiB (vs 7,3 MiB) and needs 14M on disk (vs 23M). While I like the proposal, I prefer to skip it for now. It might confused those who're used to upgrade via git fetch && git checkout x.y.z afterwards