Update notice for the piece development documents

Thank you as always for using Riiiver. We have made the following updates:

• Updated PieceJSON Generator to follow the latest specification.

• Updated the document, "PieceJSON Keys."
  Added the explanation for the new JSON keys:
  personalInfoList,
  pieceCapability,
  permissionList, and
  x-optional.

• Added the new document, "Piece Development Guideline."
  Clarified the policy for the description of PieceJSON.

• Added the new document, "Statement of the use of personal information on the PieceJSON."
  Detailed explanation about the personalInfoList mentioned above. You can learn how you should deal with the personal information.

If you need some help, please feel free to contact us at https://riiiver.com/en/contact/

We hope you continue to explore and enjoy the evolving Riiiver.