Pull Request for Issue [no issue created beforehand] .

GDPR will be in effect from may 25th, 2018.
One of the implications of this is, that a controller (see GDPR §4.7) is required to inform the 'data subject' about his/her rights regarding the collected personal data "at the time, when this data is collected" (see GDPR §13 https://gdpr-info.eu/art-13-gdpr/ ).

To help website administrators with this requirement, we could include a link to the respective "information article" in the registration form.
I implemented such a solution with this pull request (and am looking forward to discuss this).

Summary of Changes

1. include three more fields in the [backend] com_users "config.xml"
2. prepare the link to the respective article in [frontend] com_users/views/registration/view.html.php
3. add the link to the registration form, directly before the submit button in [frontend]
   com_users/views/registration/tmpl/default.php
   4 and 5) provide nessessary language strings in backend and frontend

Testing Instructions

Go to "user administration" [Backend] and open the [Advanced] tab in the user configuration dialog
provide respective info in the last three (new) configuration fields
Goto frontend and register a new user:

Expected result

the link to the "information regarding GDPR §13" shows up before the submit button
clicking the link opens a new window with this information article

Actual result

currently there is no support in Joomla to deal with this new legal requirement (although a website admin could of course always circumvent this himself).

Documentation Changes Required