Update documentation

README.adoc

@@ -18,6 +18,7 @@ Features
 Most stuff is there.  I've been using the program exclusively for many years.
 Among other things, it can display and change PulseAudio volume directly
 to cover the use case of remote control, it has a fast spectrum visualiser,
+it can be extended with plugins to fetch lyrics or other song-related info,
 and both its appearance and key bindings can be customized.
 
 Note that currently only the filesystem browsing mode is implemented,

nncmpp.adoc

@@ -128,6 +128,19 @@ For this to work, *nncmpp* needs to access the right PulseAudio daemon--in case
 your setup is unusual, consult the list of environment variables in
 *pulseaudio*(1).  MPD-compatibles are currently unsupported.
 
+Info plugins
+------------
+You can invoke various plugins from the Info tab, for example to look up
+song lyrics.
+
+Plugins can be arbitrary scripts or binaries.  When run without command line
+arguments, a plugin outputs a user interface description of what it does.
+When invoked by a user, it receives the following self-explanatory arguments:
+_TITLE_ _ARTIST_ [_ALBUM_], and anything it writes to its standard output
+or standard error stream is presented back to the user.  Here, bold and italic
+formatting can be toggled with ASCII control characters 1 (SOH) and 2 (STX),
+respectively.  Otherwise, all input and output makes use of the UTF-8 encoding.
+
 Files
 -----
 *nncmpp* follows the XDG Base Directory Specification.
@@ -135,6 +148,14 @@ Files
 _~/.config/nncmpp/nncmpp.conf_::
 	The configuration file.
 
+_~/.local/share/nncmpp/info/_::
+_/usr/local/share/nncmpp/info/_::
+_/usr/share/nncmpp/info/_::
+	Info plugins are loaded from these directories, in order,
+	then listed lexicographically.
+	Only the first occurence of a particular filename is used,
+	and empty files act as silent disablers.
+
 Reporting bugs
 --------------
 Use https://git.janouch.name/p/nncmpp to report bugs, request features,