Added section on hashtags and placeholder for groups

configs/user.config.php

@@ -25,7 +25,8 @@ $config["output_dir"] = "rendered/user";
 $config["pages"] = array(
   array("id" => "what_is_gnu_social", "title" => "What is GNU social"),
   array("id" => "getting_started", "title" => "Getting Started"),
-  array("id" => "exploring", "title" => "Exploring the Network")
+  array("id" => "exploring", "title" => "Exploring the Network"),
+  array("id" => "tags", "title" => "Hashtags and Groups")
 );
 
 ?>

html/user/tags.html

@@ -0,0 +1,41 @@
+{HEADING:SECTION:hashtags:Hashtags}
+
+{HEADING:SUBSECTION:hashtags_using:Using hashtags}
+
+<p>Hashtags are a simple idea: if you put a # in front of a word like <tt>#this</tt> then that word becomes a clickable link. When you click it you are shown messages from everyone that contain that same <em>hashtag</em>. It allows you to quickly find others who were posting about the same topic, or to bring your post to the attention of those who are watching the tag. Twitter turned the concept into a household name and it's now a staple of microblogging.</p>
+
+<img src="{RES:typing-hashtag.png}" alt="Typing a hashtag in a post" class="figure" />
+
+<p>Click on the link in a post to see all posts that include the tag:</p>
+
+<img src="{RES:showing-hashtag.png}" alt="Browsing a hashtag stream" class="figure" />
+
+<p>Note that this cannot show posts from the entire GNU social network. This limitation is explained more in the next section.</p>
+
+<p>For network-wide conversations about particular topics, see the section about {LINK:groups}. Groups perform a similar function to hashtags and they use a <tt>!</tt> instead of a <tt>#</tt>.</p>
+
+{HEADING:SUBSECTION:hashtags_limits:The limited reach of hashtags}
+
+<p>Hashtags are somewhat limited in GNU social because your server does not have a complete view of the network. Suppose your server has 10 accounts on it. Obviously it knows about every post that those 10 people make. If each of those people follow 10 different people on remote servers, that's 100 extra people. All together your server knows about the posts from 110 accounts.</p>
+
+<p>If you click on a hashtag on your server, you're limited to those 110 people. Another way to think about it is this: take the <i>Entire Known Network</i> and filter it to only include posts that contain the hashtag. That's what you'll see.</p>
+
+<p>Note also that hashtags are linked to the server where you posted. If your account is on quitter.no and you use the tag <tt>#newyearseve</tt>, anyone who clicks on it will see quitter.no's list. If somebody on quitter.se uses the same tag, clicking on that tag will show you quitter.se's list. These lists might be different because they have different views of the network.</p>
+
+<p>The GNU social federation has lots of benefits but you can see that it makes hashtags a little complicated. {LINK:groups} are a popular alternative that are more reliable.</p>
+
+{HEADING:SUBSECTION:hashtags_subscribing:Hashtag subscriptions}
+
+<p>You can subscribe to hashtags on your own server. This means that any time your server sees a post containing the hashtag, that post will appear in your <i>Home</i> timeline regardless of whether you normally follow the person who sent it.</p>
+
+<p>The button to subscribe appears when you click on a hashtag. Note that this is not visible if you are using Qvitter (i.e. any of the "quitter" servers). You can access it by switching to the "classic" mode.</p>
+
+<img src="{RES:subscribe-hashtag.png}" alt="Subscribing to a hashtag" class="figure" />
+
+<p>Once you are subscribed, not only will those posts appear in your home timeline, but you will also get quick access to the tag in your left menu. From there you can unsubscribe again.</p>
+
+<img src="{RES:subscribed-tag.png}" alt="Subscribed to a hashtag" class="figure" />
+
+{HEADING:SECTION:groups:Groups}
+
+TODO.