Document Local Development

## what
* Document how to refer to a local file rather than a remote artifact

## why
* When using `terraform-external-module-artifact` together with lambdas, sometimes for local development it's convenient to refer to a local file rather than an artifact

## how to

Just pass the `url` input with something like `file:///my/home/directory/lambda.zip`

```
module "example" {
source = "git::https://github.com/cloudposse/terraform-external-module-artifact.git?ref=master"
filename = "test.zip"
git_ref = "example"
module_name = "terraform-external-module-artifact"
module_path = "${path.module}"
url = "file:///tmp/artifact.zip"
}
```

Use the module example in the issue as the reference entry point. Document that the url input accepts a file:/// path for local artifacts, including the shown lambda.zip scenario and example. Done means the local-development procedure and example are present in the project's documentation.

Written by the indexing model from the issue text.