how_to_update_fork_ repository

图文步骤查看：
https://github.com/irene-coming/gitskills/issues/1

步骤概要：
step1. 进入fork的项目，点击 new pull request
https://user-images.githubusercontent.com/6801133/42563454-daf40b9e-8530-11e8-8e38-15e67b3ea641.png

step2. 点击create pull request, 如果按钮不可见，尝试修改来源和发起方后再重新改回最初的来源和发起方
https://user-images.githubusercontent.com/6801133/42563321-90bc7534-8530-11e8-895d-45ad3906b1b9.png

step3.点击merge pull request
https://user-images.githubusercontent.com/6801133/42563526-0cc5a3b2-8531-11e8-8a3f-034088986cbf.png

step4.点击Confirm merge
https://user-images.githubusercontent.com/6801133/42563583-2f3ce41e-8531-11e8-82f4-18dcad65cbbb.png