typo

Signed-off-by: Mattias Andrée <maandree@operamail.com>
add overview

Signed-off-by: Mattias Andrée <maandree@operamail.com>
add invoking

Signed-off-by: Mattias Andrée <maandree@operamail.com>
add rationale

Signed-off-by: Mattias Andrée <maandree@operamail.com>