Add documentation for kolab_delegation

Reviewers: #roundcube_kolab_plugins_developers

Differential Revision: https://git.kolab.org/D177

plugins/kolab_delegation/helpdocs/en_US/settings.rst

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+.. _settings-delegation:
+
+**********
+Delegation
+**********
+
+This is the place where you can give another person (in your organization) a responsibility
+for your mailboxes, calendars or task lists. Delegated person will have access to selected folders
+(write or read-only) and will have possibility to act in your behalf, e.g. reply to email,
+create events/tasks, handle invitations.
+
+To add a new user delegation use "Add delegate" button, type user name in the form on the right
+(wait for auto-completion and select the user from the list) and select folders to share.
+
+You can modify access rights at any time, remember to use *Save* button.
+
+When removing delegation you'll be asked if you want to remove also access rights to folders
+that previously were given access to. So, you can remove delegation but keep folder permissions
+for this person.
+
+.. note::
+  You can always change folder permissions in *Settings > Folders*.
+
+.. important::
+  The delegation feature to fully work requires from the delegated user to re-login.
+  In this process new identities will be created on delegated user account. They will
+  be removed when the delegator removes the delegation (depending on server configuration).