improvement(cofig.ts): throw an error when ./dist is used as output…

[SanderElias]

…path in angular.json

We will run into trouble when an user uses './dist` without subfolder. This PR does docuent that,
and also make scully check, and throw en error when this is detected

re #56

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/scullyio/scully/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Other... Please describe:

What is the current behavior?

Scully will not copy in assets, and other "strange" behavior might happen when ./dist is chosen as outputPath in angular.json.

What is the new behavior?

Documented this behavior, and put in a check inside config.ts to throw an error when this setting is found

Does this PR introduce a breaking change?

• Yes
• No

[Villanuevand]

LGTM!