document ldap basics/concept especially implemented for linuxmuster.net v7

[cweikl]

Write down some basics concerning the implemented LDAP concept used in linuxmuster.net

• basic concept
• core issues of the used ldap tree
• hoe to define special ldap filters
• usage of ldap.tools (apache) for filter definitions and debugging
• ...

[cweikl]

there is also an old issue addressing this for sophomorix: #131