upgrade/static - mention webpack setup problem

[petebacondarwin]

We need to indicate, somewhere in the docs, that there is a workaround needed before you can import the upgrade/static library into a webpack build.

See https://github.com/angular/angular/issues/13137

[jvanhees]

I second this, it took me a full day to finally fix this by using the alias. For future references: I got the error "Uncaught SyntaxError: Unexpected token export" in @angular/upgrade/static , line "export { downgradeComponent } from './src/aot/downgrade_component';"

[AlexandreHagen]

Same to me !