readme: Add link to up-to-date user guide and mark wiki as deprecated.

README.md

@@ -16,7 +16,9 @@ https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Suricata_Instal
 User Guide
 ==========
 
-https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Suricata_User_Guide
+You can follow the [Suricata user guide](http://suricata.readthedocs.io/en/latest/) to get started.
+
+Our deprecated (but still useful) user guide is also [available](https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Suricata_User_Guide).
 
 
 Contributing