Document how to start with a local file

[sharils]

Description

Document how to start with a local file so that people know that it's possible.

Motivation and Context

This is required because I didn't know it was possible so was thinking perhaps I should use swagger-ui + browser-sync and my favourite editor to do it. Now with this documented, we can get rid of all those hassles

How Has This Been Tested?

I tested this locally.

Screenshots (if appropriate):

This looks like a normal pet shop example file, but it's actually a modified version that only have some user APIs.

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.

[shockey]

thanks @sharils!