Set up ssh connection to GitHub

GitHub_ssh_connect

The reason why `git push` always failed for a forked `git_training` repository is due to multiple account settings in the same computer.
Aren't you curious why does the git not ask you about the password?
I don't spend more to dive into the problem. Just use `ssh` instead.

The following links are the steps about how to set up the ssh connection.
```
https://help.github.com/articles/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent/
```
```
https://help.github.com/articles/adding-a-new-ssh-key-to-your-github-account/
```