code: function appendToFile(fileContent, filePath) {
    return new Promise((resolve, reject) => {
      fs.appendFile(filePath, fileContent, 'utf8', (err) => {
        if (err) {
          reject(err);
        } else {
          resolve(filePath);
        }
      });
    });
  }
docs: Append a string to a file, if the file doesn't exist it is created
  @param {string} fileContent the payload of the file
  @param {string} filePath path and filename: where store the file
  @returns {Promise<string>} resolve with the filePath received in input