Added contribution instructions

Instructions to install symfony/serializer dependency was missing.
minor #911 Added contribution instructions (PapyDanone)

This PR was merged into the 1.2.x-dev branch.

Discussion
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Added contribution instructions

Instructions to install symfony/serializer dependency was missing.

Commits
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e552300 Added contribution instructions
Allow to override the access denied handler
Rework to allow a separate access denied handler per listener (type)
make $accessDeniedHandler parameter optional for BC