fix(help): doc for corporate proxy configuration with backend proxy (#4041)

[sinedied]

Following the discussion on #4041, here's a doc addition proposal when using a backend proxy behind a corporate proxy.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[Brocco]

Please remove this from the readme and add a new markdown file here: https://github.com/angular/angular-cli/tree/master/docs/documentation/stories

[hansl]

Also need to sign the CLA. Thanks!

[sinedied]

I signed it!

@Brocco will do, should I link to this file from the readme or just leave it like that in the stories/ folder?

[googlebot]

CLAs look good, thanks!

[sinedied]

I made the changes, without any link in the readme for the meanwhile

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}