Added info about JsonFileLoader added in 2.4
feature #3428 Translation - Added info about JsonFileLoader added in 2.4 (singles)

This PR was merged into the 2.4 branch.

Discussion
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Translation - Added info about JsonFileLoader added in 2.4

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#8534)
| Applies to    | 2.4
| Fixed tickets | n/a

Commits
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adf678b Added info about JsonFileLoader added in 2.4