feat(readme): add questionary link

README.md

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+
+[Who uses ngx-admin?](https://github.com/akveo/ngx-admin/issues/1645)
+
 # Admin template based on Angular 5+, Bootstrap 4 and <a href="https://github.com/akveo/nebular">Nebular</a>
 <a target="_blank" href="http://akveo.com/ngx-admin/#/pages/dashboard?utm_source=github&utm_medium=ngx_admin_readme&utm_campaign=demo"><img src="https://i.imgur.com/XoJtfvK.gif"/></a>