code: func (client *Client) GetRichMenu(richMenuID string) *GetRichMenuCall {
  	return &GetRichMenuCall{
  		c:          client,
  		richMenuID: richMenuID,
  	}
  }
docs: /*
  {
    "richmenus": [
      {
        "richMenuId": "{richMenuId}",
        "size": {
          "width": 2500,
          "height": 1686
        },
        "selected": false,
        "areas": [
          {
            "bounds": {
              "x": 0,
              "y": 0,
              "width": 2500,
              "height": 1686
            },
            "action": {
              "type": "postback"
              "data": "action=buy&itemid=123"
            }
          }
        ]
      }
    ]
  }
  */
  // GetRichMenu method