Document 20KB requirement in BandwidthRate documentation

This fixes the last suggestion of bug #1195.

doc/tor.1.txt

@@ -69,7 +69,9 @@ Other options can be specified either on the command-line (--option
 **BandwidthRate** __N__ **bytes**|**KB**|**MB**|**GB**::
     A token bucket limits the average incoming bandwidth usage on this node to
     the specified number of bytes per second, and the average outgoing
-    bandwidth usage to that same value. (Default: 5 MB)
+    bandwidth usage to that same value.  If you want to run a relay in the
+    public network, this needs to be _at the very least_ 20 KB (that is,
+    20480 bytes). (Default: 5 MB)
 
 **BandwidthBurst** __N__ **bytes**|**KB**|**MB**|**GB**::
     Limit the maximum token bucket size (also known as the burst) to the given