Merge pull request #214 from yulingtianxia/gh-pages

update advanced operators
add contributing file
Merge pull request #215 from yankuangshi/develop

add contributing file
make gitbook
make gitbook
make  gitbook
make gitbook
update Protocols
update Protocols
fix confilicts
migrate to gitbook
correct the url
Update 21_Protocols.md

表达有误
Update 21_Protocols.md

另一处表达不准确
Merge pull request #222 from zhousak/gh-pages

协议章节中关于类属性和类方法的定义描述不达意，理解有困难
Update 21_Protocols.md

忘了删掉原来的翻译～
Merge pull request #223 from zhousak/patch-1

Update 21_Protocols.md
change cover pic
Fixed #224: Typos in chap 2.16
Update 08_Enumerations.md

'显示' -> '显式'