Towards the end of the movie, after the big threat is defeated, The Hulk leaves The Avengers in frustration, as he feels no one trusts him.
After the credits! The Hulk (or Bruce Banner) walks sullenly down a lonely country road, when a regal voice says; "Mr Banner, may I have a moment?"
Hulk/Banner turns, curiously, as the voice continues; "I understand you have left The Avengers, because they treated you badly. But don't let this one experience turn you against the idea of working with others."
The speaker is revealed as DOCTOR STRANGE, who says; "I have a few ideas about a team of my own."
END