Clarify emitDecoratorMetadata requirement in getting-started

[aldenquimby]

Follows #77

Fully addresses lukeautry/tsoa#1443

Why consider this change?

• emitDecoratorMetadata has significant consequences for output JS
  • TypeScript-only changes (e.g. changing type signature, import vs import type, etc) are no longer build-only risks. They can now impact runtime code when you have this setting enabled
• emitDecoratorMetadata is particularly troublesome with isolatedModules, which is required by projects like esbuild, vite, swc, babel, next.js, and create-react-app
• see this eslint bug thread for an example deep dive into problems
• because of these complexities, any library that uses decorators should ideally clarify if emitDecoratorMetadata is required or not