Add documentation for targeting external user ids

[studnitz]

Until now this nice feature was undocumented and I was lucky to find it by diving into the source code.

Now others won't have to do the same anymore in the future. :)

[LKaemmerling]

Thank you!

[studnitz]

Thanks for merging! I would love if you'd consider adding the topic hacktoberfest to your repo or adding the label hacktoberfest-accepted to this issue. :) More Details