Add some instruction to help the review process of new content

[htwangtw]

Suggestions:

Add instruction on

• how to clone another user's remote to local client
• build the local website from a branch
• what to check (all links should be working, style, relative path within website etc)

[htwangtw]

git related stuff
https://stackoverflow.com/questions/5884784/how-to-pull-remote-branch-from-somebody-elses-repo