README for src/backend/replication/logical

Hi all

As part of some internal training/discussion I wrote some notes on logical
decoding's structure and flow that seemed worth polishing up into a draft
README for src/backend/replication/logical .

When learning this code I found that while the individual pieces are
documented it's pretty hard to understand how it all fits together, what
the data flow is, etc. Maybe this'll help the next guy.

CC'ing Andres so he can tell me how utterly wrong it all is ;)

I included an explanation of LSNs in the README because it's important to
understand them from the perspective of xlog decoding and logical
replication. It's not really the right place so I'd be happier pointing to
a comment somewhere appropriately placed instead. Same with xlog records.
The rest is pretty specific to the area of interest.

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services