added ex3.1~3.5
added ex3_6~11
added ex3.12_3.13
added ex3.14_3.20
merge the upstream
fixed some details
Merge remote-tracking branch 'upstream/master'
added ex3.21~3.23
replenish the code of previous commit.
Merge remote-tracking branch 'upstream/master'
merge @Mooophy 's clever version
edit README
Update README.md
added ex3.24~3.26
added ex3.27~3.29
Update index.md
add ex3.30~3.33
added ex3.34~3.40
Finish ch3:ok_hand:
Try to changed the color of background.