Added 2.3 upgrade notes containing app_dev.php changes regarding debug component.
merged branch pinkeen/master (PR #561)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #561).

Discussion
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Added 2.3 upgrade notes

After upgrading to symfony-2.3 I noticed that the debugging was crippled. I could not find anything in the upgrade notes, so here it is. I am not sure if I phrased it correctly.

PS Yes, I know, I should've subscribed symfony blog ages ago. I did so now.

Commits
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bdad65c Added 2.3 upgrade notes containing app_dev.php changes regarding debug component.
udpated the CHANGELOG for 2.3