removed observer pattern, in favour of mediator
[EventDispatcher] tweaked README
minor #9661 Removed observer pattern, in favour of mediator (fabpot)

This PR was merged into the 2.2 branch.

Discussion
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Removed observer pattern, in favour of mediator

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | *
| Fixed tickets | #9344

I don't think this is the best tool to implement the observer pattern.

I just have copied the sentence from:
https://github.com/symfony/symfony-docs/edit/2.3/components/event_dispatcher/introduction.rst

but maybe could be improved with something like:

"Symfony Event Dispatcher is a tool that allows to make your projects truly extensible, facilitating the implementation of Mediator pattern and Publish subscribe pattern"

[edit-1]
I think this could be modified also here:
https://github.com/symfony/symfony-marketing/edit/master/views/en/get_started/components.html.twig

Commits
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ab3e231 [EventDispatcher] tweaked README
b9ad62e removed observer pattern, in favour of mediator