Software Development Method is changing

We have seen lot of software development methodologies in last few year.
Mostly changed due to web development from client server development.
I also using the RAD technology throughout the SDLC but now it too changing very fast by AGIL method.
Now customer want their ever changing business logic very fast and in vary short span. Even some times a business logic stayed only for a week. So web engineer required new development method the answer is AGIL.
But a problem also arising due to this methodology as there are very little and almost none documentation making developer critical if not impossible to remember the logic of process and modules some times later. We can change our fast development criteria with little bit of compromise by write good code communication.