Mention the characters that are allowed in a username in the section on the add-user.sh script in the Configuration Guide

When describing how to use the add-user utility, we should mention the characters that are allowed in a username:

Usernames can only contain the following characters in any number and in any order:

• Alphanumeric characters (a-z, A-Z, 0-9)
• Dashes (-), periods (.), commas (,), at (@)
• Escaped backslash
• Escaped equals (\=)