lost code
Fixed the wrong translation.

Fixed the wrong translation of its.
update optional chaining

1.“自判断”改为“可选”
2.“声明符号”改为“叹号”
3.“本是”改为“本身”
4.一些语法语义调整
update type casting

1.cast应该翻译为转换，check才是检查
2.Protocols Checking for Protocol Conformance多写了一个Protocols
3.“下转”改为“向下转型”比较好
4.少量语法改动
make gitbook
make gitbook
add translator info
add translator info
Merge pull request #168 from yulingtianxia/develop

update optional chaining
Merge pull request #166 from twlkyao/patch-1

Fixed the wrong translation.
Update README.md

add to 02 -17 optional chaining.
fix conflict
Merge branch 'limingit-gh-pages' into gh-pages
Merge branch 'gh-pages' of https://github.com/numbbbbb/the-swift-programming-language-in-chinese into gh-pages
make gitbook
Merge pull request #169 from Jasonbroker/patch-1

Update README.md
correct the wrong "half-closed loop"
modify pattern code style
a trivial change
correct attributes grammar