Add a clarification to the doc pre-requisite namespace

[benhosmer]

The helm install fails if a flagger namespace doesn't already exist.

[stefanprodan]

@benhosmer we require commit signoff, see https://github.com/fluxcd/flagger/pull/847/checks?check_run_id=2057997095

[shubhamsre]

Wouldn't it be better to use --create-namespace while doing a helm install.