libpurple signals tutorial

[William Ehlhardt]

Since the libpurple signal interface is more or less inscrutable, I
have written a tutorial about its use. I would appreciate a second
pair of eyes checking to make sure it is correct and understandable.

It may be found in revision 2889db1b35dd79448e1222dc82a5e8022a765111
on the im.pidgin.soc.2007.certmgr branch, in the file
doc/SIGNAL-HOWTO.dox

Thanks,
-William