Merge pull request #76 from walzer/master

add contributors guide, update root/readme.md
Change the layout of the CCHttpClient article

1. Add cpp mark
2. Make subtitles easier to be seen
Fixed broken link to github repository.
Fix a typo in gui/overview/en.md
Merge pull request #78 from bunnyhero/readme-link-fix

Fixed broken link to github repository.
Merge pull request #77 from Pisces000221/patch-1

Change the layout of the CCHttpClient article
Modified `Scheduler and Timer Callback' article

added a few formattings and corrected some `fatal errors' (for example, the word 'seconds' was 'milliseconds' before)
Merge pull request #79 from Pisces000221/patch-3

Modified `Scheduler and Timer Callback' article
exchange android_on_win7 an on_mac entries
add title to android_on_mac/zh.md