gst/playback/: Updated README.

gst/playback/: Updated README.

Original commit message from CVS:
* gst/playback/README:
* gst/playback/gstplaybasebin.c: (group_destroy), (group_is_muted),
(add_stream), (unknown_type), (add_element_stream), (no_more_pads),
(probe_triggered), (preroll_unlinked), (new_decoded_pad),
(gst_play_base_bin_change_state), (gst_play_base_bin_found_tag):
* gst/playback/gstplaybin.c: (gen_vis_element), (remove_sinks),
(setup_sinks):
* gst/playback/gststreaminfo.c: (gst_stream_info_set_mute),
(gst_stream_info_is_mute), (gst_stream_info_set_property):
* gst/playback/gststreaminfo.h:
Updated README.
Only switch groups if all streams have muted (EOSed).
Send Tags in sync with the stream playback instead of in
the playback/preroll phase.
Some cleanups, free the fakesrc elements.