Improving Ada support on FreeBSD and in the ports system

[Rodrigo (ros)]

Hi porters,

Some users experience problems upgrading Yabb form 2.1 to 2.4
because they don't really read the Yabb install procedure carefully.

Even upgrade isn't destructive, I think it will be good to add those few 
lines in
the UPGRADING file to prevent users.

----
  AFFECTS: users of www/yabb
  AUTHOR: rodrigo at bebik.net

  Yabb upgrade from 2.1 to 2.4 requires a clean install before upgrade.
  See here: http://codex.yabbforum.com/YaBB.pl?num=1200162021
----


- Rodrigo