Adding nullable indicator to Describe

I'd like to add information about whether or not a parameter or result can
potentially be NULL to the RowDescription message.

Reason: I have the same reasons that Richard Jones did in 2006 (
http://archives.postgresql.org/pgsql-interfaces/2006-01/msg00043.php). I'm
writing a Haskell library that derives the types of the result at
compile-time. For nullable columns, it needs to know in advance to wrap the
type in the Maybe monad. Looking at the table column information is great,
but I'd like to support the case where a function is used in the SELECT
(among any other potential cases).

I'm new to the code base. Can you point me in the right direction?

I'd like to add the indicator without making the protocol
backwards-incompatible. My first thought was to use a bit in the type
modifier field, but that doesn't seem safe. Any suggestions?