2.15 release notes: Add upgrade info for accounts in NoteDb
2.15 release notes: Add upgrade info for accounts in NoteDb

* Link to the documentation about accounts in NoteDb.
* Say that account data is migrated automatically.
* Inform that the branches with the account data must be replicated to
  Gerrit slaves.

Change-Id: I4aa5e5e812db9ce583e92212bbf2404de2c5979f
Signed-off-by: Edwin Kempin <d43fb6c98c97e553b671ed9a9318100f1de386e7@google.com>

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