Update changlog for 2.2.2

Update changelog for 2.2.1

Tweaks on the CHANGELOG

Merge pull request nelmio#303 from gquemener/feature/named-constructor

Support instanciation of object with named constructor

Merge pull request nelmio#301 from bendavies/static-constructors

fix static constructor support

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Merge pull request nelmio#278 from Soullivaneuh/default-instance

Manage default instance without any properties

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Merge pull request nelmio#188 from tshelburne/2.0-docs

Added new documentation for 2.0

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