Update readme with --force example

[matkosoric]

I had a problem with automating aws-nuke due to interactive providing of account alias.

Solution is already written on Stackoverflow, consisted in using --force flag: Question

Still, it would be nice to have it documented directly in readme.

[jamesgeddes]

This should be returned as part of aws-nuke --help. I'd suggest is it is put under the heading **Danger Zone**.

[shibumi]

Hi, is there a way to ignore the missing alias? I have two completely different payer accounts for prod and stage... I don't want to create an alias for my stage payer account that could create problems... I don't want to touch the infra at all. I just want to nuke the resources in the accounts.

[jamesgeddes]

@shibumi Would your question be better in a separate ticket?

[shibumi]

@jamesgeddes yes, sorry for the noise. Feel free to mark the comments as irrelevant