Learn git by doing.
there will be lots of mistakes, but I will make progress.

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github suports HTTPS and SSH wich is faster and native suport by git,
so I add the ssh key to github.

ssh is prefered.
1.safe
2.faster
3.don't have to type the username & pwd everytime.
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I still typed the username and pwd last commit, beacuase 
I didn't swich remote URLs from https to ssh.