Describe different parts of the security.yml
minor #161 Describe different parts of the security.yml (ghostika)

This PR was squashed before being merged into the 3.0.x-dev branch (closes #161).

Discussion
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Describe different parts of the security.yml

Add comments for the sections of security.yml in case of the use of AcmeDemoBundle
Original issue: https://github.com/symfony/symfony-standard/issues/676

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5801688 Describe different parts of the security.yml