Symfony cheatsheet

[oxygensend]

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This PR covers issue #1222.

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In the document I covered the most common vulnerabilities and presented good practices to increase the security of Symfony apps listed in the issue. I decided to leave topics like API Rate Limiting, Encryption which are to general and not strict to Symfony. I also tried to make the scheme not to differ from cheat sheets from Ruby on Rails and Laravel.
This is my first CS and pr in opensource world, so feel free to comment, I am open for any proposals of changes :)

[edg84]

Symfony_Cheat_Sheet.md

[jmanico]

This is very solid work, I'd like this to go live soon.

[szh]

I have no knowledge of Symfony so I won't review the content, but here's quick formatting nit.