Added some documentation for set_rotate

doc/HttpSetMiscModule.wiki

@@ -748,6 +748,38 @@ For now, there's no way to configure a custom random generator seed.
 
 Behind the scene, it makes use of the standard C function <code>rand()</code>.
 
+== set_rotate ==
+'''syntax:''' ''set_random $value <from> <to>''
+
+'''default:''' ''no''
+
+'''context:''' ''location, location if''
+
+'''phase:''' ''rewrite''
+
+Increments <code>$value</code> but keeps it in range from <code>$from</code> to <code>$to</code>. 
+If <code>$value</code> is greater than <code>$to</code> or less than <code>$from</code> is will be 
+set to <code>$from</code> value.
+
+Only non-negative numbers are allowed for the <code><from></code> and <code><to></code> arguments.
+
+When <code><$from></code> is greater than <code><$to></code>, their values will be exchanged accordingly.
+
+For instance,
+
+<geshi lang="nginx">
+	location /rotate {
+	    default_type text/plain;
+	    set $counter $cookie_counter;
+	    set_rotate $counter 1 5;
+	    echo $counter;
+	    add_header Set-Cookie counter=$counter;
+	}
+</geshi>
+
+then request <code>GET /rotate</code> will output next number between 1 and 5 (i.e., 1, 2, 3, 4, 5) on each
+refresh of the page. This directive may be userful for banner rotation purposes.
+
 == set_local_today ==
 '''syntax:''' ''set_local_today $dst''