Update README with information on Allow/Deny List functionality

[dixonwille]

It is unclear what an empty allow and deny list do. README should reflect this.

[ianlewis]

Currently it seems with golangci-lint 1.53+ that an empty allow (e.g. no depguard config) means that nothing is allowed. Pretty sure that changed from 1.52 as we didn't encounter errors before.

[dixonwille]

#49

Yes the default behavior changed. Having no depguard config, but enabling it did absolutely nothing in depguard v1.

[maqdev]

Is there a way to allow everything by default, and only deny what's in deny list? Removing allow section doesn't help.

[dixonwille]

#70

I believe this is what you are looking for. I still want to document it before merging it in.