Document support of irssi

docs/Changelog

@@ -1,3 +1,6 @@
+pisg (0.23)
+   * Support irssi logfiles
+
 pisg (0.22) - Sat, Sep 15th 2001
    * pisg.cfg now supports regexs in user.cfg when 'regexp_aliases' is
      enabled. (thanks, Phil Gregory)

docs/FORMATS

@@ -45,3 +45,8 @@ pisg supports a variety of logfile formats, these are:
   - C-Based IRC Bot (http://darksun.com.pt/mbot/)
 
   - In order to use this, format must be set to 'mbot'
+
+* irssi:
+  - console IRC client (www.irssi.org)
+
+  - In order to use this, format must be set to 'irssi'