chore(starter-project): Avoid confusions on distribution approaches

[christian-bromann]

This patch resolves some of the confusions around starting a component design system with Stencil and consuming its components. Changes in particular:

• removed es2015 and es2017 as these export references aren't used anymore outside nor did they become a standardized approach to export modules
• set customElementsExportBehavior to 'auto-define-custom-elements' so that importing components doesn't need a defineCustomElement call
• set externalRuntime to false to we can move @stencil/core into the dev dependency section (fixes #95)
• add documentation around the purpose of src/index.ts
• add documentation in the README.md how to distribute the component

Will keep the PR as a draft until I updated the docs on distributing components.