Add upgrade instructions for the LoggerInterface
minor #10644 Add upgrade instructions for the LoggerInterface that changed in Symfony 2.2 (ch3ric)

This PR was submitted for the 2.2 branch but it was merged into the 2.3 branch instead (closes #10644).

Discussion
----------

Add upgrade instructions for the LoggerInterface that changed in Symfony 2.2

Commits
-------

6daa295 Add upgrade instructions for the LoggerInterface