Docs: Add explanation of `package_dir=.terraform-serverless` artifact directory default.

It's likely that the vast bulk of folks familiar with serverless would assume I think that `package_dir` should default to `.serverless`, so we should explain why the default behavior won't work and you should choose our default or something that is _not_ `.serverless`.

- [ ] Explain in README why `.serverless` artifact directory won't work because of how this provider works with the serverless lifecycle.
- [ ] Note in README we default to `.terraform-serverless` but users can configure `package_dir` for whatever they want.