Add _custom_id_ tag configuration instructions

[fridaystreet]

I've bneen searching for days to try and find how to get serverless to do this. Not saying possibly it wasn't obvious in hindsight, but certainly a lot of people have posted about using variosu plugins to tag things and that serverless tags don't get applied etc etc.

So I figured it might be well suited to actually be in the doco as it's extremely straight forward (once you know how) and it's pretty much a game changer. Localstack is unuasable as a dev tool in our environment without fixed local urls.

So apolgies if this PR isn't inline with policy etc, please feel free to do whatever you want with it. Hopefully it saves someone else 2 days

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