add chapter 2.3.4
fix __ and some sentence
Merge pull request #11 from Otsuka-iOS-Study-Club/add_chapter2.3.4-2.3.9

Add chapter 2.3.4
add chapter 2.3.5
Merge pull request #12 from Otsuka-iOS-Study-Club/add_2.3.5

add chapter 2.3.5
add 2.3.6-2.3.7
Merge pull request #13 from Otsuka-iOS-Study-Club/add_2.3.6-2.3.7

add 2.3.6-2.3.7
add chapter 2.3.8-2.3.9
Merge pull request #14 from Otsuka-iOS-Study-Club/add_2.3.8-2.3.9

add chapter 2.3.8-2.3.9
Add .md files for chapter 3
Add links for .md files in chapter 3
Update section 3.1
Merge pull request #15 from bourneshell/prerelease

Updated section 3.1