[ic] Document on writing payment modules?

[interchange-users@icdevgroup.org (Donal H.)]

Hi,

I'm using IC 4.8.5

Is there any documentation about writing new payment modules? One of our
merchants already has a relationship with a bank and wants us to use them if
possible.

I've looked through the documentation but haven't found much on this
subject. I've only found very specific questions on the mailing lists.

Thanks,
-Donal