Add new user documentation

Signed-off-by: Florian Pritz <bluewind@xinu.at>

docs/users.txt

+# Add a new dev/TU
+
+ - Add the user to group_vars/all/archusers.yml
+ - Copy the ssh key to pubkeys/$username.pub
+
+ - Run `ansible-playbook -t archusers playbooks/*.yml` or similar
+
+ - To create a new user in archweb use: https://www.archlinux.org/devel/newuser/
+   This is also linked in the django admin backend at the top
+
+ - For email accounts refer to docs/email.txt