Right now read preference is very briefly mentioned in the intro the arch guide. It is also briefly discussed in mongo/src/client/README.md. But it would be good to expand on this and discuss things like:

• how and when external clients communicate read preference (possibly include links to driver specs where appropriate)
• how and when internal clients, including mongos, communicate read preference 
• how we handle race conditions with server state changing after it is selected based on a read preference