Added the section for external PCM plugin.
<LI>Page \ref mixer explains the mixer controls API.
<LI>Page \ref pcm explains the design of the PCM (digital audio) API.
<LI>Page \ref pcm_plugins explains the design of PCM (digital audio) plugins.
+ <LI>Page \ref pcm_external_plugins explains the external PCM plugin SDK.
<LI>Page \ref rawmidi explains the design of the RawMidi API.
<LI>Page \ref timer explains the design of the Timer API.
<LI>Page \ref seq explains the design of the Sequencer API.