Complement docu regarding LDAP config

[newur]

Hello Activiti Team,

I just stumpled into a little pitfall while configuring the LDAP bean. I think it would be nice if you add a line in the user guide (and the example there) where you mention how to get the user's mail address.

For example:

<property name="userEmailAttribute" value="mail" />
‍‍‍

Without this line the class LDAPUserManager will always fail to set the EmailAttribute.

Best regards and many thanks for your great work
Ruwen

[jbarrez]

Here you go https://github.com/Activiti/Activiti/commit/40c480099df1207b505bf0545803ffc2c4af206e