added a note about is* getters support with GetSetMethodNormalizer
feature #3603 [Serializer] Support for is.* getters in GetSetMethodNormalizer (tiraeth)

This PR was submitted for the master branch but it was merged into the 2.5 branch instead (closes #3603).

Discussion
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[Serializer] Support for is.* getters in GetSetMethodNormalizer

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#10314)
| Applies to    | 2.5+
| Fixed tickets | N/A

Commits
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fcfcfb9 [Serializer] added a note about is* getters support with GetSetMethodNormalizer