[gexiv2] Announcement: Minor incompatibilities with master

[Jens Georg <mail jensge org>]

Hi,

 - The API of the vala bindings of Metadata.from_app1_segment() and
   Metadata.open_buf() has changed. Both take now an proper "array"
   instead of an array and a length

 - The meson options have been changed to be less "autotoolish":
    - enable-gtk-doc -> gtk_doc
    - disable-introspection -> instrospection
    - disable-vala -> vapi