sticker: added guidelines for sticker names

doc/sticker.xml

+<?xml version='1.0' encoding="utf-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+               "docbook/dtd/xml/4.2/docbookx.dtd">
+<book>
+  <title>The Music Player Daemon Sticker Database</title>
+
+  <chapter>
+    <title>Introduction to MPD's Sticker Database</title>
+    <para>
+      This document shell give a short guideline for recommended tags
+      for use in MPD's Sticker Database.
+      MPD's Sticker Database is a subsystem that enables users to add
+      custom tags. MPD does not alter the media files.
+    </para>
+  </chapter>
+
+  <chapter>
+    <title>Guideline for recommended tags</title>
+    <para>
+      Since there is no standard for tags in media files, this
+      document is trying to give you some help deciding what tags to
+      use.  The selection of these tags tries to cover the most
+      widely used tags.  This way the tags might still work in other
+      players, if you sync the database with your original media
+      files.
+    </para>
+
+    <tag>
+      <name>Rating</name>
+      <value>1-5</value>
+      <para>
+        Will store a rating value from 1 (worst) to 5 (best) for a given song.
+      </para>
+    </tag>
+
+    <tag>
+      <name>Album Rating</name>
+      <value>1-5</value>
+      <para>
+        Will store a rating value from 1 (worst) to 5 (best) for a given album.
+      </para>
+    </tag>
+
+    <tag>
+      <name>Style</name>
+      <value>Keyword</value>
+      <para>
+        This tag is used to keep the Genre tag clean, by now having 1000's of genres.
+        Instead you define a Main Genre for each file and can make a more specific
+        description. This should be one Keyword like "Post Punk" or "Progressive Death Metal"
+        An Alternative name for this tag is "Subgenre", time will tell which one gets
+        more support.
+      </para>
+    </tag>
+
+    <tag>
+      <name>Lyrics</name>
+      <value>The lyrics of the song, including header with Artist - Title</value>
+      <para>
+        This one is self explaining. This gives the option to store lyrics of
+        a song where they belong to: mapped to the song
+      </para>
+    </tag>
+
+    <tag>
+      <name>Similar Artists</name>
+      <value>Comma seperated list of artists</value>
+      <para>
+        This tag enables a last.fm alike aproach which will still work when being offline
+        Keep in mind, that this tag is absolutely non-standard! I am not aware of any
+        other player that uses a comparable tag.
+      </para>
+    </tag>
+  </chapter>
+</book>