Update doc for project permissions and groups

doc/groups.markdown

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+Groups Management
+=================
+
+In Kanboard, each users can be member of one or many groups.
+A group is like a team or an organization.
+
+Only administrators can create new groups and assign users.
+
+Groups can be managed from **User management > View All Groups**.
+From there, you can create groups and assign users.
+
+![Group Management](screenshots/groups-management.png)
+
+Each project manager can authorize the access to a set of groups from the [project permissions page](project-permissions.markdown).
+
+The external id is mainly used for external group providers.
+Kanboard provide a LDAP group provider to [sync automatically groups from LDAP servers](ldap-group-sync.markdown).