README hint: @typings/core-js errors when consuming library

[real-marco-b]

What I did:

1. Build and publish/pack angular-library-seed npm
2. Clone https://github.com/angular/quickstart as library consumer project.
3. Integrate published library via npm into consumer project
4. "npm run build" consumer project

Getting loads of typescript errors regarding typings/core-js. Seems like a core-js dependency conflicts with Es6 definitions distributed with typescript.
As a workaround, I downgraded es2015 lib to es5 lib within tsconfig.json:

"lib": [ "es5", "dom" ],

instead of:

"lib": [ "es2015", "dom" ],

This worked for me.

This is not a problem with the angular-library-seed itself, but consuming the library from within an angular 2 quickstart clone should be a common scenario.
This information might be an interesting addition to README.md or some other troubleshooting doc :)